As new infectious diseases emerge, the development of vaccines is essential in boosting immunity in the population. Understanding the properties of vaccines is important from a public health perspective to better optimize immunization strategies and forecasting. However, in the early stages of vaccine rollout it can be difficult to estimate these properties, specifically the duration of vaccine protection.
CEID’s Pejman Rohani worked with a research team to design a simulation study to examine how accurately the duration of vaccine immunity against emerging pathogens can be estimated with age-specific case report incidence data. The team simulated a study period of one year following the introduction of COVID-19 vaccines in Germany. The simulation model used for this study is an extension of the standard Susceptible-Exposed-Infected-Recovered (SEIR model), which places individuals into one of these four categories based on exposure and illness status. The team categorized individuals according to age and type of infection, either clinical (symptomatic) or subclinical (asymptomatic). Subclinical infections were assumed to be half as infectious as the clinical infectious, consistent with estimates from the first COVID-19 wave in China.
This model incorporated realistic elements of disease transmission by including latent and infectious periods. Contact rates between age groups were fixed using data from a 2019 United Kingdom study of 7,290 participants. Vaccination was assumed to be staggered and continuous in the population. Further, the model assumed that 85% of those aged seventy years or older would get the vaccine as well as 85% of those aged sixty to sixty-nine, 65% of those aged twenty to fifty-nine, and 50% of those aged ten to nineteen years old. A mean duration of protection of .85 years (rapid waning) or two years (slow waning) was specified.
For slow waning vaccines, the results showed that estimation of the duration of vaccine immunity was usually imprecise and inaccurate. For rapid-waning vaccines, the average duration of protection estimates were consistently accurate for six months of daily data. Vaccine heterogeneity, variance in immune response, was determined to be an independent predictor of vaccine performance estimation. Across the vaccines tested in the model, rapid-waning and high heterogeneity vaccines allowed for duration of protection to be completed for all vaccines while slow-waning and low heterogeneity vaccines were only able to estimate duration for a fraction of vaccines with rapid immunity loss. To predict the impact of mass immunization campaigns for emerging diseases, it is important to conduct further research in this field and consider the heterogeneity of vaccine protection.
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By: Brenna Daly