Efficacy of prior infection with Omicron and booster vaccination against subvariant BA.5

The update bivalent recall has been shown to be effective against COVID-19, and in particular against severe disease progression. However, few studies have been conducted on the protective effect of Omicron infection followed by a booster vaccination.

A new studypublished yesterday in Open JAMA Networksought to estimate the protective efficacy of a prior COVID-19 BA.1 or BA.2 infection and a booster vaccination against the Omicron BA.5 subvariant.

The national population-based study was conducted in South Korea and used a nested risk-sampled case-control design. Study patients and at-risk controls were selected from the Korea COVID-19 Vaccine Effectiveness (K-COVE) dataset. The cohorts were matched on age, sex and place of residence.

The patients in the study had all tested positive for COVID-19 during the BA.5 prevalence period (August 1-31, 2022). Cases and controls who had never contracted COVID-19 were proportionally compared to patients who had previously had BA.1 or BA.2 infection.

Vaccine efficacy was estimated using a logistic regression model to adjust for covariates such as health status, vaccination status, and infection history. Vaccine efficacy against BA.5 infection and severe infection was compared in patients with no prior infection, prior BA.1 infection, and prior BA.2 infection.

A total of 3415980 cases and 3415980 controls were included in the study. The cohorts were made up of 53.9% women and were on average 40.2 years old. In people with no history of COVID-19, the efficacy of the 4-dose vaccine (2 injections for a primary series, 1 booster and 1 bivalent booster) was estimated at 16.1%. The efficacy of the vaccine was 89.5% in people with a previous BA.1 infection and 94.3% in people with a previous BA.2 infection.

Against critical BA.5 infection, the efficacy of the 4-dose vaccine was 90.9% in infection-naïve patients, 93.9% in those previously infected with BA.1, and 92. 9% in people who have already been infected with BA.2.

It is not surprising that a prior COVID-19 BA.1 or BA.2 infection, together with a bivalent booster vaccination, is highly protective against the Omicron BA.5 subvariant. As previous studies have shown, the 4-dose vaccination is much more effective than the 2-dose, especially against critical COVID-19 infection.