COVID-19 Vaccination Plus Previous Infection Most Protective Against Reinfection

Results of a case-control study, published in The Lancet Infectious Diseases, suggest previous COVID-19 infection plus vaccination provides the most robust immunity against reinfection among adolescents.

Researchers from the United Kingdom Health Security Agency and National Institute for Health and Care Research sourced data for this observational, test-negative, case-control study from national health databases. They estimated the risk for symptomatic COVID-19 infection following variant-specific primary infection and vaccine-specific vaccination using polymerase chain reaction testing data collected between August 2021 and March 2022 from adolescents aged 12 to 17 years. The primary outcome was protection against COVID-19 Delta and Omicron infection, defined as 1 – odds of vaccination or previous infection in cases divided by odds of vaccination or previous infection in controls. Logistic regression models were used to estimate protection conferred by combinations of vaccination and previous infection.

The overall study population comprised 1,161,704 individuals, among whom 390,467 tested positive for Delta infection, 212,433 tested positive for Omicron infection, and 558,804 tested negative for infection. The percentage of individuals who were unvaccinated was highest among those infected with the Delta (79.2%) variant, followed by those who were negative for infection (55.0%) and those who were positive for Omicron infection (40.9%). In addition, the highest rate of previous COVID-19 infection was observed among individuals who were negative for infection (14.6%) compared with those who were either positive for infection with the Omicron (13.5%) or Delta (1.2%) variants.

Previous infection with wild-type virus was associated with an estimated protection rate of 87.6% against reinfection with the Delta variant and 32.7% against reinfection with the Omicron variant. For the Alpha and Delta variants, previous infection was associated with an estimated protection rate of 86.1% and 36.6% against Delta and 92.3% and 52.4% and Omicron reinfection, respectively. Previous Omicron infection was associated with an estimated protection rate of 59.3% against reinfection with Omicron.

After receipt of the first BNT162b2 vaccine dose, the estimated rate of protection against infection ranged from 8.5% to 59.4% among individuals with no previous COVID-19 infection. After receipt of the second vaccine dose and a booster dose, the rate of estimated protection ranged between 71.0% and 91.8% and 84.8% and 96.0%, respectively. The estimated rate of protection conferred by any mRNA vaccine booster dose ranged between 33.6% and 62.9%.

For individuals previously infected with wild-type virus, a single BNT162b2 vaccine dose protected against reinfection at an estimated rate of 92.6% to 98.6%, 90.3% to 95.5% for those with previous Alpha variant infection, and 91.3% to 99.0% for those with previous Delta variant infection. Similar trends were observed after the second BNT162b2 vaccine dose.

The highest estimated rate of protection against Omicron infection was observed among individuals who were both previously infected with the Omicron variant and had received the second vaccine dose within the previous 15 to 24 weeks (96.4%).

These study results may be limited as previous infection was defined as the most recent infection, however, some individuals may have had serial infections.

According to the researchers, “[O]ur findings provide important evidence of only modest short-term protection against mild disease with omicron variant infection following vaccination…”