The importance of supplementary immunisation activities to prevent measles outbreaks during the COVID-19 pandemic in Kenya

Background: The COVID-19 pandemic has disrupted routine measles immunisation and supplementary immunisation activities (SIAs) in most countries including Kenya. We assessed the risk of measles outbreaks during the COVID-19 pandemic in Kenya as a case study for the African Region. Methods: Combining measles serological data, local contact patterns, and vaccination coverage into a cohort model, we predicted the age-adjusted population immunity in Kenya and estimated the probability of outbreaks when contact-reducing COVID-19 interventions are lifted. We considered various scenarios for reduced measles vaccination coverage from April 2020. Findings: In February 2020, when a scheduled SIA was postponed, population immunity was close to the herd immunity threshold and the probability for one case to initiate a large outbreak as 22% (0-46). As restrictions to physical contact are lifted, from December 2020, the probability for a large outbreak increased to 31% (8-51), 35% (16-52) and 43% (31-56) assuming a 15%, 50% and 100% reduction in vaccination coverage. By December 2021, this risk is higher at 37% (17-54), 44% (29-57) and 57% (48-65) for the same coverage scenarios respectively. However, the increased risk of a measles outbreak following the lifting of the COVID-19 restrictions on contact can be overcome by conducting an SIA with ≥ 95% coverage in under-fives. Interpretation: While the reduction in measles transmissibility through COVID-19 restrictions temporarily reduced the risk from a measles immunity gap, this risk rises again rapidly once physical distancing is relaxed. Implementing delayed SIAs will be critical for prevention of measles outbreaks once contact restrictions are fully lifted in Kenya.