Impact of booster vaccination interval on SARS-CoV-2 infection, hospitalization, and death

We investigated how booster interval affects the risks of SARS-CoV-2 infection and Covid-19-related hospitalization and death in different age groups.

We collected data on booster receipts and Covid-19 outcomes between September 22, 2021 and February 9, 2023 for 5,769,205 North Carolina residents ≥12 years of age who had completed their primary vaccination series. We related Covid-19 outcomes to baseline characteristics and booster doses through Cox regression models.

For adults ≥65 years of age, boosting every 9 months was associated with proportionate reductions (compared with no boosting) of 18.9% (95% CI, 18.5-19.4) in the cumulative frequency of infection, 37.8% (95% CI, 35.3-40.3) in the cumulative risk of hospitalization or death, and 40.9% (95% CI, 37.2-44.7) in the cumulative risk of death at 2 years after completion of primary vaccination. The reductions were lower by boosting every 12 months and higher by boosting every 6 months. The reductions were smaller for individuals 12-64 years of age.

Boosting at a shorter interval was associated with a greater reduction in Covid-19 outcomes, especially hospitalization and death. Frequent boosting conferred greater benefits for individuals aged ≥65 than for individuals aged 12-64.