Effectiveness of second booster compared to first booster and protection conferred by previous SARS-CoV-2 infection against symptomatic Omicron BA.2 and BA.4/5 in France.

In face of evidence of rapid waning of vaccine effectiveness against Omicron and its sub-lineages, a second booster with mRNA vaccines was recommended for the most vulnerable in France. We used a test negative design to estimate the effectiveness of the second booster relative to the first booster and the protection conferred by a previous SARS-CoV-2 infection, against symptomatic Omicron BA.2 or BA.4/5. We included symptomatic ≥60 years old individuals tested for SARS-CoV-2 in March 21-October 30, 2022. Compared to a 181-210 days old first booster, a second booster restored protection with a relative effectiveness of 41% [95%CI: 39-42%], 7-30 days post-vaccination. This gain in protection was lower than the one observed with the first booster, at equal time points since vaccination. High levels of protection were associated to previous SARS-CoV-2 infection, especially when the infection was recent and occurred when an antigenic-related variant was dominant.