The College of Surgeons of East, Central, and Southern Africa (COSECSA) training program requires completion of a mandatory rotation in vascular and cardiothoracic surgery. However, few accredited training sites offer such services regularly, hence exposure of trainees to these disciplines is limited. This study evaluates the demand, feasibility, and acceptance of an open-access, synchronous virtual surgical education series to bridge gaps in training.

The Center for Equity in Global Surgery at the University of Global Health Equity partnered with COSECSA to develop and deliver interactive sessions on common cardiothoracic, vascular, and transplant surgical topics facilitated by faculty from the COSECSA region and global partner institutions. All sessions were delivered via a cloud-based video conferencing service and recorded for retrospective viewing. At the end of the lecture series, trainees participated in a post-course evaluation survey.

In total, 2,015 participants from 48 countries attended the lecture series with 977 participants in 2022 and 1,038 in 2023. In 2023, most participants were from Ethiopia (10.9%), Kenya (23.6%), and Uganda (15.4%). Two hundred and fifty-two participants contributed to the post-course evaluation survey, of which only 23% were female and 48% were general surgery trainees. Mean satisfaction with the lecture series on a 5-point Likert scale was 4.5. Participants reported an average satisfaction level of 4.4 for logistics of the series (registration process, ease of connectivity, and time management), 4.5 for content, 4.4 for in-class interactions, 4.4 for use of case-based images, and 4.2 for post-session availability of lecture material. Almost all suggested additional lectures in thoracic infection, vascular trauma, and plastic surgery to be included in subsequent lecture series.

The study has shown that such an online, synchronous, virtual teaching series is in high demand, feasible, and crucial for closing knowledge gaps in didactic cardiothoracic, vascular, and transplant surgery within the COSECSA region. These sessions are well received by COSECSA trainees and are accessible, highlighting a clear demonstrable need. It is recommended that such efforts be sustained and expanded, with additional competency areas to broaden the impact.

An inadequate number of specialist surgical providers in sub-Saharan Africa constitutes a significant barrier to safe, timely, affordable, equitable, and quality healthcare. While surgical workforce deficits are global, they are more acute in sub-Saharan Africa. In contrast to the recommended global targets of 20 surgical, anesthesia, and obstetrics specialists per 100,000 population, sub-Saharan Africa has only 2 per 100,000, and Eastern, Central, and Southern Africa host only 0.53 surgeons per 100,000 population [1]. The College of Surgeons of East, Central, and Southern Africa (COSECSA) was founded in 1999 to address gaps in surgical training and to enhance the quality of surgical education and clinical practice in the region [2]. COSECSA training prioritizes practical experience, mentorship, and ongoing professional growth.

The training pathway under COSECSA comprises a membership program designed to inculcate the understanding of the basic principles of surgery, and a fellowship specialty qualification in general surgery, orthopedics, urology, pediatric surgery, otorhinolaryngology, plastic surgery, pediatric surgery, cardiothoracic surgery, and neurosurgery in addition to sub-specialty qualifications in pediatric orthopedic surgery and cardiothoracic surgery. While exposure to training in surgical oncology, vascular, cardiothoracic, and transplant surgery is mandatory for trainees, few accredited COSECSA training sites consistently offer such services due to low specialist surgeon densities and infrastructure challenges in the region. The limited number of expert trainers and sub-specialist capacity in teaching facilities implies that exposure of trainees to such disciplines has been limited historically.

To fill this gap, COSECSA and the Center for Equity in Global Surgery (CEGS) at the University of Global Health Equity (UGHE) partnered to co-design and deliver a 12-week, open-access, synchronous, virtual multidisciplinary lecture series from 2021 to 2023. The education series focused on surgical topics to which trainees have limited exposure across the subspecialties of surgical oncology, cardiothoracic surgery, and transplant surgery. The purpose of the education series is to enhance surgical education and collaboration across the East, Central, and Southern Africa region by leveraging technology, expert faculty, and participant feedback to address relevant topics in cardiothoracic, vascular, and transplant surgery. The educational partnership engages faculty from the COSECSA region and global partner institutions to facilitate sessions, ensuring diverse perspectives and high-level expertise from Low-Income and High-Income Country contexts.

As part of the educational program audit and quality improvement, this study assessed trends in feasibility, demand, and acceptance of this open-access, online surgical education program targeted at multidisciplinary learners from a variety of African Low- and Middle-Income Countries (LMICs). We specifically assessed the educational outcomes and outputs of the program on the first level of the Kirkpatrick framework with an emphasis on barriers and facilitators to engagement and track course attendance and attendee distribution over the first two years of the program. This study also evaluates the demand, feasibility, and acceptance of an open-access, synchronous virtual surgical education series to bridge gaps in surgical training.

The study is a retrospective review of course evaluation and audit data. Comprehensive course evaluation surveys were developed and administered to participants at the end of the lecture series to gather feedback on the effectiveness of the sessions, quality of content, facilitation, and overall learning experience. The feedback received was used to inform adjustments and improvements for subsequent education sessions and for future iterations of similar programs. The quality improvement data was received over two years between 2021 and 2023.

COSECSA is a regional training surgical college comprising 14 constituent member countries [3] and nine partner countries (Fig. 1). The college aims to enhance surgical services within Africa through various programs by increasing the number of appropriately trained specialist surgeons and surgically trained general medical officers in the East, Central, and Southern Africa regions [3]. The college has 142 accredited training centers in 19 countries [4], and specialists trained mainly in lower-level, rural district facilities, which may lack sub-specialty expertise. Coordinated from its headquarters in Arusha, Tanzania, COSECSA graduated a total of 758 surgical specialists as of December 2022 [3].

Map Showing COSECSA member countries including partner countries

Full size image

The Center for Equity in Global Surgery at the University of Global Health Equity was set up in 2022 to design solutions for issues in global surgical care in the African setting. Based on the pillars of education and training, policy and advocacy, research and innovation, fellowships, and global convenings, the center works to dismantle surgical education barriers through inclusivity, innovation, partnerships, co-location of regional training hubs, and utilizing a multidisciplinary, trans-sectoral approach [5].

Using Smith’s model of curriculum theory and practice with a focus on content, values and process, praxis/awareness, and product [6] (Fig. 2), the course was co-developed and iteratively modified by a committee of eight surgical educators via a series of preparatory meetings before its launch.

Smith’s Model of Curriculum Theory and Practice (license: https://uxwing.com/license/)

Full size image

Content

To identify the knowledge gaps and training requirements in surgical specialties across sub-Saharan Africa, a needs assessment was conducted to guide the development of course content. The team then employed a topic selection framework that considered the following factors: relevance to the sub-Saharan African context, limited exposure to the topics at training facilities, identified needs from COSECSA examination results, and trainee requests. This led to the development of course content that focused on surgical oncology, vascular surgery, cardiothoracic surgery, and organ transplantation (Fig. 2). The team ensured that the content was contextualized to the sub-Saharan African setting, taking into account regional challenges, resources, and healthcare systems.

Values

The program values of inclusivity and appropriateness of gaze and pose were co-defined and operationalized during the design stage by identifying facilitators from both High-Income Countries and LMICs who had strong African contextual expertise, enabling culturally sensitive and relevant content delivery. The process of course delivery was agreed on as online synchronous lectures with virtual, real-time interaction via a cloud-based video conferencing service. Session dates and times were set for every Wednesday for 90 min from 1:30 pm East African Time from June through August. Sessions were recorded and made available to participants for review, promoting flexibility and self-paced learning.

Praxis/awareness

The praxis/awareness at the design stage involved practical deliberation on the use of technology, recognizing the limitations that the model of instruction would provide for the delivery of a differentiated curriculum and the need for cross-cultural publicity. The team collaborated with stakeholders and subject experts to co-design publicity materials that were culturally sensitive and relevant. These materials were distributed via email and social media platforms, ensuring broad reach and accessibility. The distribution plan identified target audiences for promotion, including potential participants, stakeholders, and partners.

Technology and product

The team utilized technology that had already been pre-tested by UGHE in the wake of COVID- 19, ensuring a smooth transition to an online educational model. The selected technology allowed for scalability and flexibility, enabling the program to adapt to changing needs and circumstances. The key product of the course was to equip surgical providers within the sub-region with the fundamental knowledge and skills in vascular/cardiothoracic, oncology, and transplant surgery. This was achieved to enhance the diagnostic capabilities of individuals with limited exposure to these specialties. We also planned for course evaluation as a yearly output to shape curriculum content review and choice of examiners for subsequent years.

The steering committee meticulously selected faculty to ensure the highest standards of education and diverse expertise. The faculty team comprised sub-specialists in the priority training areas and experienced professionals, with an equitable distribution from the COSECSA region and the global community. This volunteer-based faculty had been oriented on their roles and responsibilities during preparatory meetings, ensuring they were well-prepared for their sessions. We maintained regular follow-ups and reminders to ensure no faculty member missed their scheduled sessions, guaranteeing consistent and high-quality instruction for COSECSA learners.

In operationalizing the sessions, a poster was prepared to advertise each lecture in the series and widely distributed by the Center for Equity in Global Surgery and COSECSA to prospective attendees (see Fig. 3). Interested individuals pre-registered for the online lecture series. Table 1 shows the topics and the facilitators.

Social media advert of the lecture series on COSECSA 'X' page

Full size image

A retrospective review was performed of the lecture series and post-session evaluation data from participants who participated in the COSECSA lecture series between 2022 and 2023. Based on session registration data, a report was generated and downloaded from the cloud-based platform. Data related to the number of participants, time spent in each session, countries of attendance for each lecture, and the most viewed topics or lectures were collected from these reports. See Additional File 1 for the registration datasheet template.

Data was collected by a trained data collector, and quality control was performed by the data manager for errors to ensure clean and quality data was collected for analysis. A Microsoft Excel file was designed as a database for the storage of collected data, including data from previous training sessions.

An online survey at the end of the lecture series in 2022 was prepared using the CHERRIES methodology [7], to obtain feedback from attendees on the acceptance of the sessions, quality of content, facilitation, and overall learning experience. Attendees were asked to complete a post-session evaluation survey, the link was shared with attendees after the lecture series. The survey consisted of 12 individual questions with an additional feedback section where the attendees could leave their comments and ask any additional questions that they may have. See Additional File 2 for the post-session evaluation questionnaire. The course evaluations were primarily for internal audit and quality improvement. Data were collected on a secure Google form containing open-ended questions on Kirkpatrick Level 1 (perception of the sessions) focused on how to improve the standard and quality of the training sessions. The level of satisfaction with timing, session length, registration, facilitators, and the use of case-based imaging, among other elements, was assessed on a 5-point Likert scale. Results were anonymous to guarantee an objective evaluation.

Statistical analyses were performed using JASP (Version 0.18.3) [Apple Silicon]. We evaluated trends in attendance using measures of central tendency and simple proportions. We also evaluated the perception of learners around acceptance, barriers, facilitators, participants’ satisfaction, perceived value, and areas for improvement using simple proportions. Inductive content analysis of the open-ended survey responses was conducted based on grounded theory and constant comparison to extract nuanced insights and recommendations for future training sessions. Specifically, the topic detection/categorization technique was used. This technique involves grouping similar relevant themes by categorizing the text into several similar categories.

This data was collected as part of internal audit and quality improvement, considering that the study exclusively relied on existing data collected as part of educational activities for retrospective audit. Therefore, the study received an exemption from the UGHE Health Research Ethics Committee with ref. UGHE-IRB/2024/343-EXE. All respondents gave documented informed consent prior to filling out evaluation forms.

The findings of the lecture series from two different periods were considered: 2022 and 2023. Twenty-four consecutive lectures in the series were analyzed, with eight lectures from 2022 and 16 lectures from 2023.

A lecture series was held over two years, 2022 and 2023, see Table 2. In 2022, 977 participants from 36 countries joined, with the majority coming from Ethiopia (31.7%), Kenya (17.8%), and Uganda (9.0%). The following year, 2023, saw an increase in participants to 1,038 from 37 countries, with Kenya, Uganda, and Ethiopia remaining among the top contributors. Notably, several countries, including Congo, Lesotho, and Mozambique, showed significant increases in participation.

The lecture series saw a slight increase in participation from 2022 to 2023, with a broader global reach. Notably, Ethiopia had the most participants in 2022, while Kenya led in 2023, (See Fig. 4). Participation expanded from four continents in 2022 to six in 2023, with new representation from South America and Australia. Participation from Africa increased from 69.4% in 2022 to 78.4% in 2023 (Fig. 5).

World Map of Attendance: Year 2022 (left) and 2023 (right)

Full size image

Charts Showing the Participation from Continents

Full size image

Table 3 highlights the distribution of the participants by specialty for the two lecture periods. Across the two time frames, the main participant specialty was general surgery (12% in 2022 and 48.5% in 2023). Among the participants in 2022, a total of 119 (12.2%) were general practitioners, while in 2023, 236 (22.7%) were at the Membership (MCS) level of training with COSECSA. A brief statistical comparison across specialties was performed using a Sign test, and the results revealed no significant difference in the representation of specialties (p = 0.3616).

Among the countries in the 2022 time frame, significant participation was noted from 12 different countries, including Burundi, Eswatini, Ethiopia, Kenya, Malawi, Rwanda, Sudan, Tanzania, Uganda, USA, Zambia, and Zimbabwe, with participants attending all eight lectures in the series. In 2023, attendance for all the lecture series was noted for six countries, including Kenya, Angola, Congo, Eswatini, Sudan, and Tanzania, with participants from these countries attending all 16 lectures in the series.

The number of participants for each lecture series significantly increased between the two periods (Fig. 6). An average attendance of 210 (standard deviation = 9.68) was reported among the eight lectures in 2022, whereas a mean attendance of 223.5 (standard deviation = 57.8) was registered over the 16 lectures in 2023. Over the 2022 lecture series, a stable number of attendees was maintained, ranging from 163 to 240. A more dynamic attendance was reported during the 2023 lecture series, ranging from 163 to 317. A chi-square test was conducted to assess whether the observed increase in attendance was statistically meaningful. The results of the Sign test revealed a non-significant p-value of 0.5601, indicating that the observed increase in attendance from 977 in 2022 to 1,038 in 2023 (+ 6.2%) is not statistically significant.

Attendance for each Lecture Series

Full size image

A total of 252 participants completed the post-session evaluation in 2022; the demographic characteristics of participants are seen in Table 4 below.

Considering the Kirkpatrick Model’s level 1 evaluation, using the five-point self-assessment scale ranging from ‘very dissatisfied’ to ‘very satisfied’, the majority of the participants expressed satisfaction. Specifically, across both years, 110 participants (43.7%) reported being satisfied, while 124 participants (49.2%) indicated being very satisfied with the primary outcome of the lecture series (Fig. 7).

Level of Satisfaction of Participants

Full size image

More than 78.6% of overall survey respondents felt that the lectures would contribute positively to their exam preparation. Regarding future attendance of the COSECSA lecture series, 99.2% of the survey participants reported that they would attend future COSECSA lecture series (Table 5).

A ‘word cloud’ was created based on responses from the participants when asked about their favorite sessions. In order of preference, this word cloud revealed that thoracic infections, esophageal cancer, the approach to vascular trauma, and acute vascular occlusion were the most favored sessions (Fig. 8).

What attendees mentioned as their favorite sessions (wordart.com)

Full size image

At the end of the post-session evaluation, an open-ended question permitted the participants to list additional topics to be added to subsequent lecture series (Fig. 9). The four most commonly reported among the 19 topics to be included in subsequent lecture series were General Surgery, Pediatric Surgery, Plastic and Reconstructive Surgery, and Urology sessions.

Word Cloud demonstrating topics to be included in subsequent lecture series (wordart.com)

Full size image

The Synchronous Online Surgical Education Lecture Series on Cardio-thoracic, Vascular, Oncology, and Transplant Surgery was co-designed to address the inadequate number of surgical specialists as a result of educational gaps faced by surgical trainees in sub-Saharan Africa. Similarly, a study by Coyan et al. [8] documented the interest in specialized surgical fields among trainees but noted that challenges include limited exposure and the need for improved technical experience [8]. Furthermore, trainees expressed a desire for optimized training designs and showed a preference for learning through periprocedural discussions and online resources [9]. Our study findings align with a 2023 study by Alayande et al. that assessed an interactive hybrid global surgery course for cross-site interdisciplinary learners. The study concluded that online learners had positive perceptions of online learning [10].

The lecture series demonstrated a significant demand for surgical education in sub-Saharan Africa, with a notable trend of increasing attendance throughout the series. Our data indicates a 6.2% increase in attendance for the second lecture series in 2023 compared to the first lecture in 2022. This upward trend underscores a burgeoning interest and need for accessible, high-quality, specialized surgical education among trainees within the region. This finding corroborates an earlier study by Naidu et al. [11] highlighting the need for training and retention of surgeons in sub-Saharan Africa in addressing the shortages of surgeon specialists [11].

The worldwide participation of attendees is also noteworthy. Our series attracted participants from 48 different countries, with Ethiopia, Kenya, and Uganda being the top three represented countries. This widespread participation highlights the reach and accessibility of online education, bridging geographical gaps and providing equal opportunities for surgical trainees across Africa. The distribution pattern also underscores the importance of regional collaborations and the need to tailor educational programs to the specific contexts and needs of different countries. Furthermore, the majority of attendees were general surgeons and general practitioners. This reflects a critical demand among these professionals for specialized knowledge to enhance their practice and address complex surgical cases.

The presence of general practitioners highlights the broader scope of surgical practice in sub-Saharan Africa, where practitioners often encounter a wide range of medical conditions requiring surgical intervention. Each surgical specialty session exhibited unique attendance patterns. Cardio-thoracic and Oncology sessions consistently attracted the highest number of participants, reflecting perhaps the critical need for expertise in managing the increasing burden of non-communicable diseases such as cancer and cardiovascular conditions in the region. Fish et al. [12], Mcgrath et al. [13], and Taliente et al. [14] reported similar findings, showing the rising cancer burden in sub-Saharan Africa, underscoring the need for education and training, particularly in surgical oncology [12,13,14]. Vascular and Transplant surgery sessions, while initially less attended, showed a gradual increase in participants, indicating growing awareness and interest in these specialized fields. While our analysis suggests a relatively balanced representation of specialties, we acknowledge that a more detailed analysis may be necessary to identify potential areas for improvement. Future studies will consider conducting a more in-depth analysis to inform targeted efforts.

We acknowledge that the underrepresentation of women in surgical education is a critical issue that requires attention. As highlighted in a recent publication by Mulwafu et al. [2], the establishment of Women in Surgery Africa (WiSA) demonstrates a commitment to addressing gender disparities in surgery [2]. However, the limited representation of women (9%) in the surgical workforce underscores the need for continued efforts to overcome the barriers to female representation [2]. We propose a range of strategies, including mentorship programs, targeted recruitment, inclusive curriculum, and scholarship opportunities, to support the education and training of female surgical residents.

The curriculum's relevance to the current surgical care challenges and the quality of the lectures, delivered by esteemed international and regional experts, have been pivotal in attracting and retaining attendees. Building stronger partnerships with regional medical institutions, professional bodies, and international organizations enhanced the reach and effectiveness of the lecture series. The study by Mcgrath et al. [13] emphasized the relevance of international collaborative education programs in delivering quality specialty education. The study further highlights that capacity-building partnerships can improve education, thereby acknowledging the skillset and expertise of the local community. Feedback from participants highlights the relevance of the lecture contents, case-based learning, and interactive sessions. Our lecture series addresses the gap in surgical education by providing accessible and high-quality educational content, thereby enhancing the skills and knowledge of surgical trainees. The findings from our study align with the study by Jayakumar et al. [15], which reported that virtual training delivered through online tutorials can effectively teach theoretical knowledge and surgical skills [15].

The success of this lecture series has implications for the future of surgical education in sub-Saharan Africa. Online platforms offer a cost-effective and efficient way to reach a large audience, reducing barriers to education and promoting capacity building in the region, resonating with the study by Davis et al. [16]. Future directions include expanding the series to include hands-on training and mentorship opportunities, further addressing the surgical workforce shortage in sub-Saharan Africa. Strengthening partnerships with regional medical institutions, professional bodies, and international organizations to enhance the reach and acceptance of the future lecture series. Given the significant participation of general practitioners and general surgeons, future programs should continue to cater to these groups. Providing more specialized content can help these practitioners expand their skill sets, improve patient care as well as prepare them for their specialty examinations. The success of the online option of education suggests that future lecture series should continue to leverage technology to maximize accessibility. Exploring additional features such as virtual simulations and interactive workshops can further enhance the learning experience.

The virtual surgical lecture series has proven to be a valuable educational resource for surgical trainees in sub-Saharan Africa. The results of our study indicate a strong demand and positive reception for this series among the participants. The success of this lecture series has several important implications for the future of surgical education in sub-Saharan Africa. The high levels of participant satisfaction indicate the success of this initiative and underscore the importance of continuing to provide accessible, high-quality surgical education in the region.

To further enhance the virtual surgical lecture series, several key actions should be implemented. Firstly, efforts should be directed towards expanding access to the series, thereby reaching a broader spectrum of surgical trainees across sub-Saharan Africa. To sustain this initiative, strategic planning should focus on enhancing visibility, improving marketing strategies, and exploring partnerships with regional healthcare institutions for faculty recruitment and retention. In addition, the college has institutionalized a low-cost, high-yield effort in funding and has also partnered with surgical organizations and institutions and provided opportunities for faculty development and mentorship.

Furthermore, the virtual surgical lecture series should be ensured to remain a sustainable and accessible resource for surgical trainees in sub-Saharan Africa. This will involve maintaining the lecture series’ relevance, engagement, and up-to-date content, as well as exploring innovative technologies and platforms to elevate the educational experience. In addition, integrating the virtual lecture series into the curriculum of participating institutions will ensure that the program aligns with their educational goals and objectives. The annual quality improvement approach has contributed to the strengthening and maintenance of the program without compromising quality while expanding the content from 8 to 16 lecture series.

Longitudinal studies will also be conducted to assess the long-term effects of the lecture series on surgical trainees’ knowledge, skills, and patient outcomes. These studies will provide valuable insights into the series’ effectiveness and guide future enhancements. In the future, we will conduct a follow-up survey or assessment at 6 to 12 months to evaluate participants’ knowledge retention and application, specifically assessing their Kirkpatrick Level 2 (Learning) achievement. This will involve a multiple-choice questionnaire, case-based scenarios, or a practical skills assessment. For Level 3, we will conduct a study to assess participants’ behavioral changes, such as their adoption of new surgical techniques or changes in their clinical practice. This will involve a mixed-methods approach, including surveys, interviews, or observation of participants’ clinical practice. For long-term Kirkpatrick Level 4, we will conduct a study to evaluate the clinical impact of our virtual lecture series, such as improvements in patient outcomes, reduced complications, or enhanced quality of care. This could involve a retrospective or prospective cohort study, utilizing data from electronic health records or national databases.

While the virtual surgical lecture series demonstrated high participant satisfaction and engagement, further research is needed to assess its effectiveness in improving skill acquisition and clinical application. The findings provide preliminary evidence supporting the feasibility and acceptability of such a program, and we plan to incorporate more rigorous evaluation methods in future studies to substantiate claims of educational value.

CEGS:

Center for Equity in Global Surgery

CHERRIES:

Checklist for Reporting Results of Internet E-Surveys

COSECSA:

College of Surgeons of East, Central, and Southern Africa

JASP:

Jeffreys’s Amazing Statistics Program

LMICs:

Low- and Middle-Income Countries

UGHE:

University of Global Health Equity

Download references

Author notes

Conceptualization, Data Curation, Investigation, Methodology, Supervision, Visualization, Validation, Resources, Writing – Original Draft Preparation, Writing – Review & Editing: A.B., Conceptualization, Data Curation, Investigation, Formal Analysis, Visualization, Methodology, Validation, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing: B.A., Data Curation, Formal Analysis, Project Administration, Visualization, Writing – Original Draft Preparation, Writing – Review & Editing: O.O., Data Curation, Visualization, Writing – Original Draft Preparation, Writing – Review & Editing: J.I., Data Curation, Investigation, Formal Analysis, Project Administration, Writing – Review & Editing: N.B., Formal Analysis, Writing – Review & Editing: C.F., Supervision, Writing – Review & Editing: S.I., Writing – Review & Editing: D.G., M.M., P.N.M., R.R., and V.M.

Correspondence to Oluwaseun Ojomo.

The study received an exemption from the UGHE Health Research Ethics Committee with ref. UGHE-IRB/2024/343-EXE. All respondents gave documented informed consent prior to filling out evaluation forms.

Not applicable.

The authors declare no competing interests.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Reprints and permissions