Study: Absolute effectiveness of recent COVID

Lewis NM, et al. Open Forum Infect Dis. 2022;doi:10.1093/ofid/ofac698.

Lewis NM, et al. Open Forum Infect Dis. 2022;doi:10.1093/ofid/ofac698.

The absolute vaccine effectiveness of a recent first booster dose of COVID-19 messenger RNA vaccine against COVID-associated hospitalization is 81% compared with 46% for the primary series alone, researchers found.

"There are a few important considerations for why we study COVID-19 vaccine effectiveness against hospitalization, and for why we designed this study in the way we did," Nathaniel M. Lewis, PhD , a member of the CDC's COVID-19 Response Team, told Healio.

A "substantial benefit" of COVID-19 vaccines is the prevention of severe outcomes, Lewis noted, so assessing the degree to which a vaccine regimen prevents someone from getting hospitalized with severe COVID-19 is important.

Because the U.S. population is now "generally well covered" in terms of having received COVID-19 booster doses, it is important to see how effective the boosters were in preventing COVID-19 hospitalization compared with no vaccination or having received just a primary series, Lewis said.

Lewis and colleagues wrote that studies assessing COVID-19 vaccine efficacy "are increasingly reporting relative VE (rVE) comparing a primary series plus booster doses to a primary series only."

"Interpretation of rVE differs from traditional studies measuring absolute VE (aVE) of a vaccine regimen against an unvaccinated referent group," they wrote.

"Since relative VE is the percentage of the remaining effectiveness after one regimen that was ‘made up’ by having received the other regimen, the number cannot be interpreted in the same way that we interpret regular VE," Lewis said. "When we see a high relative VE estimate — for example, 90% — it means that the booster provided a great deal of additional benefit, but it did not necessarily accomplish the same reduction in the outcome (hospitalization) as was achieved by the primary series."

Lewis and colleagues assessed booster-eligible immunocompetent adults who were hospitalized at 21 medical centers in the U.S. between Dec. 25, 2021, and April 4, 2022, using a test negative design. According to the study, they used logistic regression with case status as the outcome and completion of primary vaccine series or primary series plus one booster dose as the predictors, adjusted for potential confounders, to estimate aVE and rVE.

Overall, the researchers assessed 2,060 patients, including 1,104 with COVID-19 and 956 controls. They found that the rVE against COVID-19 hospitalization in boosted messenger RNA vaccine recipients vs. those who had primary series only was 66% (95% CI, 55%-74%), whereas aVE was 81% (95% CI, 75%-86%) for boosted and 46% (95% CI, 30%-58%) for primary series alone.

Additionally, the study showed rVE was 49% (95% CI, –9% to 76%) and aVE was 62% (95% CI, 33%-79%) for boosted participants who received the Johnson & Johnson vaccine vs. 36% (95% CI, –4% to 6%) for those who received only the primary dose.

"Vaccine booster doses increase protection against COVID-19 hospitalization compared to a primary series," Lewis said. "However, comparing relative VE measures across studies can lead to flawed interpretations of the added value of a new vaccination regimen, whereas difference in absolute VE (vs. an unvaccinated group), when available, may be a more useful metric of effectiveness for each of the two regimens — booster and primary series."