The global SEARCH network: providing a framework for critical COVID-19 healthcare worker cohort studies transitioning from the pandemic to the interpandemic period

Summary box Healthcare workers (HCWs) played a critical role in the global response to the COVID-19 pandemic, providing essential frontline care while putting themselves and potentially their household members at greater risk of infection. In addition, HCWs participated in applied research, including participating in cohort studies, to help advance our understanding of SARS-CoV-2 virus transmission, COVID-19 vaccine effectiveness, barriers to vaccine uptake and other urgent public health questions. Studies have expanded to address additional public health issues, such as influenza vaccine effectiveness and hesitancy, COVID-19 disease burden, correlates of protection against infection, antimicrobial resistance monitoring, effectiveness of infection prevention and control measures and RSV disease burden. Investment in these HCW cohorts during the interpandemic period is essential to provide a framework to quickly evaluate the implementation of any new vaccines, antivirals, medications and other interventions that could be introduced in response to the next pandemic threat. Healthcare workers (HCWs) played a critical role in the global response to the COVID-19 pandemic, providing essential frontline care while putting themselves and potentially their household members at greater risk of infection.1 HCWs around the world made another important but often overlooked contribution during the pandemic; they participated in applied research, including as participants in cohort studies, to help advance our understanding of SARS-CoV-2 virus transmission, COVID-19 vaccine effectiveness, barriers to vaccine uptake and other urgent public health questions. Protecting the health of this essential population is paramount during pandemics and between pandemics, and therefore, monitoring disease and the effectiveness of preventive measures among HCWs is critical. HCWs have a high exposure risk in occupational settings, and infected HCWs may infect vulnerable patients. Ensuring a functioning healthcare system and healthcare surge capacity requires keeping HCWs healthy. HCWs can also be a convenient population to study; they are a willing and informed population, with a particular interest in participating in research that can improve patient outcomes; they are also easily accessible at their place of work, which facilitates research activities. At the onset of the COVID-19 pandemic in 2020, longitudinal HCW cohort studies were established in diverse locations globally. Tens of thousands of HCWs in hospitals and clinics, including doctors, nurses and support staff, volunteered to participate in these studies, where they routinely provided respiratory and blood samples and completed frequent questionnaires about symptoms, exposures and knowledge and attitudes about newly available vaccines. These cohort studies provided some of the first data on risk factors for SARS-CoV-2 infection and reinfection,2 COVID-19 vaccine effectiveness,3 factors associated with COVID-19 vaccine uptake in HCWs4 and correlates of protection against SARS-CoV-2 infection.5 Data were rapidly shared with government decision-making bodies and publicly through manuscripts in preprint servers and peer-reviewed journals. Findings helped shape national and international public health policy and guidelines, particularly regarding the COVID-19 vaccination strategy, throughout the pandemic.6 HCW cohort studies were not limited to high-income countries (HICs): studies evaluating COVID-19 vaccine effectiveness and safety were conducted in low- and middle-income countries (LMICs) in South America,7 Asia,8 the WHO European region9 and Africa,10 where diverse COVID-19 vaccines and platforms were used in demographically diverse populations. Although the WHO declared in May 2023 that COVID-19 was no longer a public health emergency of international concern, many HCW cohorts initiated during the pandemic remain in place. These studies continue to answer important public health questions about the effectiveness of vaccines against new variants of the SARS-CoV-2 virus as new vaccine compositions are introduced, issues related to COVID-19 vaccine uptake and vaccine hesitancy, COVID-19 disease burden and correlates of protection against infection. Studies have expanded to address other public health issues, such as influenza vaccine effectiveness and hesitancy, antimicrobial resistance monitoring, effectiveness of infection prevention and control measures and RSV disease burden. Investment in these HCW cohorts during the interpandemic period is essential to provide a framework to quickly evaluate the implementation of any new vaccines, antiviral medications and other interventions that could be introduced in response to the next pandemic threat. To provide a collaborative framework for ongoing HCW cohort studies, we have jointly established a new global network of HCW cohort studies, named SEARCH (Surveillance of Emerging Infectious Diseases and Agile Research Cohorts in Healthcare Workers), which includes representatives from the WHO, UK Health Security Agency, US Centers for Disease Control and Prevention (CDC), European CDC, Epiconcept, Sheba Hospital in Israel, Albania Institute of Public Health, Georgia National CDC, Azerbaijan Public Health and Reforms Center, Irish Health Service Executive, National Health Institute Ricardo Jorge in Portugal and US Naval Medical Research Unit SOUTH in Peru. We held our inaugural meeting in London on 6–7 March 2024. The SEARCH network aims to provide an international forum to discuss evolving objectives and new research questions related to emerging infections, support strategic approaches to operational and methodological challenges, share lessons learnt from the pandemic and support cross-study data sharing and analyses. By formalising this network of HCW cohort studies, we aim to facilitate collaborative efforts to ensure complementary activities across studies and pursue funding opportunities. Ongoing commitment and funding will be essential to ensure the continuation of these studies; with limited resources in the post-pandemic period, cross-network projects in diverse healthcare settings that include both HICs and LMICs may be more likely to gain traction with funding organisations. The SEARCH network plans to hold regular meetings with investigators in both virtual and in-person formats. We are also keen to include additional HCW cohort study teams that were not part of our first meeting. HCWs are a key resource—a population of vital importance for public health preparedness and willing partners in research. Following the critical contributions of HCW cohort studies during the pandemic and recognising the ongoing utility of these studies to inform public health policy and pandemic planning, the SEARCH network will provide a framework to strengthen collaborations among HCW cohort studies during the interpandemic period and perhaps through the next pandemic. For more information about the SEARCH network, please contact the SEARCH network leads at Europct@who.int or siren@ukhsa.gov.uk, or through the website https://surveys.ukhsa.gov.uk/siren/