The effectiveness of COVID-19 vaccination against the risk of SARS-CoV-2 infection, hospitalization, and death wanes progressively over 9 months, but the rate of waning varies according to the type of vaccine, according to research recently published in the Lancet.
Researchers conducted a retrospective, total population cohort study in Sweden, sourcing data for a total of 1,685,948 individuals from Swedish nationwide registers. All individuals immunized with 2 doses of any COVID-19 vaccine (ChAdOx1 nCoV-19, mRNA-1273, or BNT162b2; n = 842,974) and all individuals with a recorded SARS-CoV-2 infection were matched by birth year, sex, and municipality to the same number of nonimmunized individuals. Data from all participants were assessed for 2 outcomes: (1) SARS-CoV-2 infection of any severity from January 12 to October 4, 2021 and (2) severe COVID-19, described as hospitalization for COVID-19 or death from any cause within 30 days of confirmed infection from March 15 to September 28, 2021.
Vaccine effectiveness declined progressively for all vaccines types during follow-up, but waning occurred at different speeds. For SARS-CoV-2 infection of any severity, the vaccine effectiveness of BNT162b2 diminished progressively over time, from 92% (95% CI, 92-93; P <.001) at 15-30 days, to 47% (95% CI, 39-55; P <.001) at 121-180 days, and to 23% (95% CI, -2 to 41; P =.07) from day 211 onward. Effectiveness declined slightly more slowly for mRNA-1273, which had a vaccine effectiveness of 96% (95% CI, 94-97; P <.001) at 15-30 days and 59% (95% CI, 18-79; P = .012) from day 181 on. Waning also was slightly slower for heterologous ChAdOx1 nCoV-19 plus an mRNA vaccine, for which vaccine effectiveness was 89% (95% CI, 79-94; P <.001) at 15-30 days and 66% (95% CI, 41-80; P <.001) from day 121 on. In contrast, vaccine effectiveness for homologous ChAdOx1 nCoV-19 vaccine was 68% (95% CI, 52-79; P <.001) at 15-30 days, with no demonstrable effectiveness from day 121 onward (95% CI, -19% [-98 to 28]; P =.49). When examining the outcome of severe COVID-19, vaccine effectiveness waned from 89% (95% CI, 82-93; P <.001) at 15-30 days to 64% (95% CI, 44 to 77; P <.001) from day 121 forward.
Generally, some evidence showed lower vaccine effectiveness in men than in women and in older individuals than in younger ones. No detectable vaccine effectiveness was observed in men (17%; [95% CI, -13 to 40]; P =.23) from day 181 onward, whereas it endured in women (34%; [95% CI, 22-45]; P <.001). At 61-120 days, vaccine effectiveness waned to 50% (95% CI, 30-64; P <.001) in those 80 years or older.
In sensitivity analyses, the data confirmed declining vaccine effectiveness for SARS-CoV-2 infection of any severity, including the different rate of waning for different vaccine
schedules. This analysis also confirmed that vaccine effectiveness upheld better against the outcome of severe COVID-19 than against SARS-CoV-2 infection of any severity, although some waning was seen after 4 months.
The study was limited by the possibility of residual and unmeasured confounding, including a greater risk of selection bias in unvaccinated individuals with longer follow-up time. It also is likely that some participants with a prior asymptomatic infection were included in the analyses.
“The results of our study have important clinical implications, as they strengthen the evidence-based rationale for administration of a third vaccine dose as a booster, especially to specific high-risk populations,” the investigators wrote.
Nordström P, Ballin M, Nordström A. Risk of infection, hospitalisation, and death up to 9 months after a second dose of COVID-19 vaccine: a retrospective, total population cohort study in Sweden. Lancet. Published online February 4, 2022. doi:10.1016/S0140-6736(22)00089-7