Autumn 2022 COVID-19 boosters halved hospitalization and death risk in adults over 50, waning after 70 days

The autumn 2022 COVID-19 booster campaign in England administered vaccines to millions of adults aged 50 and older who had already received their primary vaccination course. Whether those boosters continued to reduce the most serious outcomes - hospitalization and death - and for how long, is the question a new study published in Vaccine addresses with a large, well-matched dataset.

The findings, from research led by the universities of Bristol and Oxford using the OpenSAFELY research platform, confirm that boosters provided substantial and sustained protection. Boosted individuals had meaningfully lower risks of COVID-19 hospitalization and death across a follow-up period of nearly a year. Protection declined over time but remained detectable across the full observation window.

Study design: 3.4 million adults, matched comparison

The NHS England-approved analysis used linked general practice and hospital records available within OpenSAFELY, a secure analytics platform built on real-world clinical data. The team identified 3,464,877 adults eligible for the autumn 2022 booster and matched each to an unboosted individual similar in age, time since last COVID-19 vaccine dose, prior vaccine brand, clinical vulnerability level, and geographic region. Matching on these factors controls for confounding that would otherwise make boosted and unboosted groups incomparable.

Across 2.5 million people followed for up to a year, the study recorded 14,436 COVID-19 hospitalizations, 1,152 COVID-19 deaths, 32,184 non-COVID-19 deaths, and 52,758 bone fractures. The fracture outcome was included as a negative control - an outcome not expected to be causally related to vaccination - to test whether the matched comparison groups were genuinely comparable on health characteristics.

The protection numbers

The core results at 350 days of follow-up: boosted individuals had COVID-19 hospitalization rates of 3.78 per 1,000 compared to 6.81 per 1,000 in unboosted individuals. For COVID-19 death, the figures were 0.29 per 1,000 (boosted) versus 0.61 per 1,000 (unboosted). Boosters approximately halved both risks across the full follow-up period.

The timing of protection mattered. Effectiveness was strongest during the first 70 days after the booster dose, after which it declined. This waning pattern is consistent with booster data from earlier in the pandemic and reflects the known behavior of vaccine-induced immune responses to respiratory pathogens. Protection against severe outcomes tends to be more durable than protection against infection, but it still erodes over time.

Moderna versus Pfizer: comparable on what matters

Both the Moderna BA.1 (mRNA-1273) and Pfizer-BioNTech BA.1 (BNT162b2) boosters performed similarly for COVID-19 hospitalization and death. This equivalence is meaningful for program planning: the two vaccines can be interchanged without meaningful differences in severe disease outcomes. One finding warrants attention: non-COVID-19 mortality was slightly higher in the Moderna group than the Pfizer group. The study authors note that the small fracture rate difference was also higher in the Moderna group, suggesting this may reflect residual confounding - systematic differences between who received which vaccine brand - rather than a causal effect of the vaccine itself. The fracture finding provides evidence that the matched design was not perfect, and some unmeasured differences between brand groups likely persisted.

What this adds to the evidence

The autumn 2022 campaign targeted Omicron BA.1 subvariants, and the bivalent vaccines used were specifically formulated against Omicron rather than original Wuhan-lineage virus. The study provides outcome data for this specific formulation in a large real-world population, complementing trial and earlier observational data. The long follow-up of nearly a year allows detection of waning that shorter studies cannot capture.

Observational data carries inherent limitations. Despite careful matching, individuals who chose to receive a booster may differ from those who did not in ways that the matching variables do not fully capture - the "healthy vaccinee" effect, where more health-conscious individuals are both more likely to seek vaccination and more likely to engage in other protective behaviors. The study cannot fully rule out residual confounding from such behavioral differences.