Abstract
Background
Single-dose vaccination was widely recommended in the pre-Omicron era for persons with previous SARS-CoV-2 infection. The effectiveness of a second vaccine dose in this group in the Omicron era is unknown.
Methods
We linked nationwide population registries in Spain to identify community-dwelling individuals aged 18-64, with a positive SARS-CoV-2 test before single-dose mRNA vaccination (mRNA-1273 or BNT162b2). Every day between January 3 and February 6, 2022 we matched 1:1 individuals receiving a second mRNA vaccine-dose and controls on sex, age, province, first dose type and time, month of primary infection and number of previous tests. We then estimated Kaplan-Meier risks of confirmed SARS-CoV-2 reinfection. We performed a similar analysis in a Delta-dominant period, between July 19 and November 30, 2021.
Results
In the Omicron period, estimated effectiveness (95% confidence interval) of a second dose was 62.2% (58.2, 66.4) 7 to 34 days after administration, similar across groups defined by age, sex, type of first vaccine and time since the first dose. Estimated effectiveness was 65.4% (61.1, 69.9) for mRNA-1273 and 52.0% (41.8, 63.1) for BNT162b2. Estimated effectiveness was 78.5% (67.4, 89.9), 66.1% (54.9, 77.5), and 60.2% (55.5, 64.8) when primary infection had occurred in the Delta, Alpha, and pre-Alpha periods, respectively. In the Delta period, the estimated effectiveness of a second dose was 8.8% (-55.3, 81.1).
Conclusions
Our results suggest that, over a month after administration, a second dose of mRNA vaccine increases protection against SARS-CoV-2 reinfection with the Omicron variant among individuals with single-dose vaccination and previously infected with another variant.
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