Syndromic surveillance and immunoprofiling of inbound travelers as a proxy for national SARS-CoV-2 trends in Uganda: implications for border health policy in resource-limited settings

BACKGROUND: This study evaluated SARS-CoV-2 infection rates among inbound international travelers to Uganda between May 2022 and July 2024, following the introduction of mandatory COVID-19 vaccination in March 2021. It also explored the relationship between traveler positivity rates and national COVID-19 trends. METHODS: We analyzed laboratory records of 4,517 inbound travelers tested at a laboratory which screens travelers at Entebbe International Airport. The testing algorithm included nucleic acid amplification testing (NAAT) for viral detection and chemiluminescent immunoassays to measure IgG antibody titers. Infection rates were stratified by vaccination status and age. To understand how vaccine- or infection-induced immunity might wane differently across age groups, impacting infection risk, we examined the interaction between IgG levels and age on PCR positivity and compared traveler positivity trends to national case data. RESULTS: The overall infection rate was 10.2% (461/4,517), with unvaccinated travelers having a significantly higher infection rate (13.1%; 253/1,935) than vaccinated ones (8.1%; 208/2,582). After adjusting for age and sex in a multivariate logistic regression analysis, vaccinated individuals had 42% lower odds of testing positive compared to those who were unvaccinated (Adjusted Odds Ratio: 0.58; 95% CI: 0.48–0.71; p < 0.001). Protective IgG antibodies were present in 93.4% (2,412/2,582) of vaccinated individuals, compared to 6.1% (118/1,935) of the unvaccinated. PCR positivity was influenced by both age and IgG levels, with a non-linear interaction. Traveler positivity remained high across the study period, even as national case counts declined post-August 2022. CONCLUSION: Vaccination significantly reduced infection risk and conferred higher IgG protection among travelers. The consistent high positivity among travelers, despite low national transmission, underscores the importance of continued surveillance at ports of entry as an early warning system, especially during global variant emergence. CLINICAL TRIAL NUMBER: Not applicable.