Long-term waning of vaccine-induced immunity to measles in England

Abstract

Background: The proportion of double vaccinated cases during measles outbreaks in England has increased since 2010, especially among teenagers and young adults. Possible explanations include: rare infections in vaccinated individuals who did not gain immunity upon vaccination, made more common as the proportion of the population born before vaccination decreases; or waning of vaccine-induced immunity, which would present new challenges for measles control in near elimination settings. Methods: To assess explanations for observed dynamics, we used a mathematical model stratified by age group, region and vaccine status, fitted to case data reported in England from 2010 to 2019. We evaluated whether models with or without waning were best able to capture the temporal dynamics of vaccinated cases in England. Findings: Only models with waning of vaccine-induced immunity captured the number and distribution by age and year of vaccinated cases. The model without waning generated more single-vaccinated cases, and fewer double-vaccinated cases above 15 years-old than observed in the data (median: 73 cases in simulations without waning, 202 in the data, 187 when waning was included). The estimated waning rate was slow (95% credible interval: 0.036% to 0.044% per year in the best fitting model), but sufficient to increase measles burden because vaccinated cases were almost as likely to cause onwards transmission as unvaccinated cases (95% credible interval for risk of onwards transmission from vaccinated cases was only 7% to 21% lower relative to unvaccinated cases). Interpretation: Measles case dynamics in England is consistent with waning of vaccine-induced immunity. Since measles is highly infectious, a slow waning leads to a heightened burden, with an increase in the number of both vaccinated and unvaccinated cases. Our findings show that the vaccine remains protective against measles infections for decades, but breakthrough infections are increasingly likely for individuals aged 15 and older. Funding National Institute for Health Research; Wellcome Trust.