Duration of Protection Against Yellow Fever After Primary Vaccination

The duration of protection conferred by vaccination against yellow fever virus (YFV) infection varies depending on age at the time of vaccination receipt and immunocompromised status, according to results of a systematic review and meta-analysis published in Clinical Infectious Diseases. 

Researchers identified 36 studies from 20 countries published between 1993 and 2019 to include in a systematic review and meta-analysis. The researchers evaluated the duration of protection conferred by 1 or 2 or more doses of any licensed yellow fever vaccine. Comparators included placebo, no vaccine, another type of vaccine, fractional or booster doses of YFV vaccine alone or with in combination with a vaccine for a related disease (eg, dengue fever; Japanese encephalitis). The included studies comprised more than 17,000 patients aged between 6 months and 85 years. Data were stratified by time since primary vaccination and assessed via random-effects models. The outcomes of interest included the number of patients with YFV infection, the rate of mortality, and the number of patients who were seropositive for neutralizing antibodies against YFV infection.

All included patients were either residents of areas in which YFV is endemic or travelers from nonendemic areas. Subgroups included children, adults aged 18 years and older, those 60 years and older, those who were immunonocompromised, pregnant women, and those who were vaccinated against other types of flavivirus.

The researchers assessed seroprotection among healthy adults and children 3 months after vaccination receipt. Results showed that the pooled rate of seroprotection was approximately 100% in both healthy children and adults and 92% in those who were immunocompromised. Although further analysis showed that pooled seroprotection rates at 5 years remained high in adults (≤97%), decreased rates were observed in children (52%) and those who were immunocompromised (86%). After 10 years, the pooled rate of seroprotection was 88% in adults, 54% in children, and 75% in those who were immunocompromised.

Among patients with autoimmune disease, the rate of pooled seroprotection 1 month following primary vaccination ranged between 73% and 85%. In patients who received 2 or more vaccine doses, seropositivity was maintained for up to 33 years following receipt of the second dose.

Among patients between the ages of 60 and 80 years, protection against YFV infection was observed for up to 10 years after primary vaccination.

Limitations include a high risk of bias, heterogeneity, and small sample sizes among the studies included in the analysis.

According to researchers, “a single dose of [YFV] vaccine confers high levels of immunity (as measured by seroprotection rates) in healthy adults for up to 10 years, after which waning occurs, thereby increasing the risk for secondary vaccine failures.”

Reference

Kling K, Domingo C, Bogdan C, et al. Duration of protection after vaccination against yellow fever – systematic review and meta-analysis. Clin Infect Dis. Published online July 20, 2022. doi:10.1093/cid/ciac580