Italian investigators conclude that the relative vaccine effectiveness (RVE) of a second booster vaccine dose against severe COVID-19 is only about 30% after 2 to 4 months in people 80 and older compared with protection after the first booster dose. The study is published in Vaccine.
The RVE of the second booster dose was negligible for general infections at 2 to 4 months for those 80 and older. The study was conducted between April and August of 2022, during a time of Omicron BA.2 and BA.5 subvariant dominance.
To conduct the study, researchers matched data from the national vaccination registry and the COVID-19 surveillance system in a 1:1 ratio of individuals who received the second booster vaccine dose with individuals who received the first booster vaccine dose at least 120 days earlier. A total of 831,555 matched pairs were analyzed.
A second booster dose of mRNA vaccine, 14 to118 days post administration, was moderately effective in preventing SARS-CoV-2 infection compared to a first booster dose administered at least 120 days earlier (14.3%, 95% confidence interval [CI]: 2.2-20.2), the authors said.
Overall RVE decreased from 28.5% (95% CI: 24.7-32.1) in the time-interval 14 to 28 days to 7.6% (95% CI: -14.1 to 18.3) in the time-interval 56 to118 days. For severe COVID, there was more protection, decreasing from 43.2% (95% CI: 30.6-54.9) to 27.2% (95% CI: 8.3-42.9) over the same time span.
"Although RVE against SARS-CoV-2 infection was much reduced 2-4 months after a 2nd booster dose, RVE against severe COVID-19 was about 30%, even during prevalent circulation of the Omicron BA.5 subvariant," the authors concluded.