CDC: Time Blunts COVID-19 Booster Shot Protection Too

Protection against COVID-19 waned 4 to 5 months after a third dose of Pfizer (Comirnaty) or Moderna’s (Spikevax) vaccine, with steeply declining protection against emergency department (ED) visits as well as hospitalization during Omicron, researchers found.

During the Omicron period, vaccine effectiveness (VE) against ED or urgent care visits was 87% in the first 2 months after receiving a third dose of mRNA vaccine, but declined to 66% after 4 to 5 months and slid to 31% after at least 5 months, reported Jill Ferdinands, PhD, of the CDC, and colleagues.

VE against hospitalization also waned during Omicron, slipping from 91% at 2 months following the third dose down to 78% at 4 months and beyond, the authors wrote in an early edition of the Morbidity and Mortality Weekly Report.

Using a test-negative design, they examined data from eight VISION Network sites, which tracked ED/urgent care visits and hospitalizations for adults from Aug. 26, 2021 to Jan. 22, 2022. The start date was 14 days after the U.S. first recommended a third dose of mRNA vaccine, at the time for immunocompromised individuals. The time periods were divided into Delta and Omicron periods, with the Omicron period starting when Omicron accounted for more than 50% of sequenced isolates per study site, they added.

Overall, there were 241,204 eligible ED and urgent care encounters, defined as “a COVID-like illness diagnosis” among adults tested for COVID via RT-PCR in the 14 days before a medical visit or in the 72 hours after. Three-quarters of these visits (77%) took place during the Delta period, with the remaining during Omicron.

During the study period, 46% were unvaccinated, 44% had received a two-dose series, and 10% had received three doses. The study could not distinguish whether individuals received a third dose as part of their primary series or as a booster, a cited limitation, though only 1,736 of the 25,138 individuals receiving three doses were listed as immunocompromised.

VE was higher during the Delta period, though it still declined with time. Ferdinands’ group found that among those who received three doses of vaccine, VE against ED or urgent care visits was 97% within 2 months of the third dose, declining to 89% after at least 4 months.

A two-dose series provided only modest protection against ED or urgent care visits during the Omicron period — 69% VE within 2 months of the second dose and 37% at 5 months or later.

Examining hospitalizations, the disparities were more evident: with 89% of 93,408 eligible hospitalizations occurring during the Delta period. Similar to ED visits, 43% were among the unvaccinated, 45% received a two-dose series, and 12% received three doses.

Protection against hospitalization during Delta was 96% within 2 months of a third dose, but dropped to 76% among those vaccinated at least 4 months earlier. However, the authors noted caveats to these findings as “few data were available on persons vaccinated for ≥4 months after a third dose during the Delta-predominant period.”

Likewise, protection against hospitalization during Omicron for those with only a two-dose primary series was 71% within 2 months of vaccination, falling to 54% among those vaccinated at least 5 months prior.