A decline in the effectiveness of the Pfizer COVID-19 vaccine (Comirnaty) against infection appeared to be due to waning immunity, not the Delta variant, researchers found.
Examining sequenced infections, protection against infection with the Delta variant was 93% (95% CI 85-97) for the first month following full vaccination but then dropped to 53% (95% CI 39-65) after 4 months, reported Sara Tartof, MD, of Kaiser Permanente Southern California in Pasadena, and colleagues.
That drop was comparable in the larger cohort, where the vaccine was 88% effective against infection (95% CI 86-89) after the first month but dropped to 47% (95% CI 43-51) after 5 months, the authors wrote in The Lancet.
The vaccine was also still effective at preventing COVID-related hospitalizations due to the Delta variant, with 93% effectiveness (95% CI 84-96) for up to 6 months, they noted.
“Protection against infection does decline in the months following a second dose,” Tartof said in a statement. “While this study provides evidence that immunity wanes for all age groups that received the vaccine, the CDC Advisory Committee on Immunization Practices [ACIP] has called for additional research to determine if booster shots should be made available to all age groups eligible for this vaccine.”
The statement added that this research confirms “preliminary reports” from both Israel and the CDC.
CDC currently recommends a booster dose of Pfizer vaccine for certain adults initially vaccinated with Pfizer, including all adults age 65 and up, those ages 50-64 with high-risk medical conditions. It says 18- to 49-year-olds with high-risk medical conditions and adults 18-64 with high occupational or institutional risk may receive boosters, as well.
The authors examined electronic health records from December 14, 2020, to August 8, 2021, in the Kaiser Permanente Southern California health system for individuals ages 12 and up. Participants had at least 1 year of prior membership in the health system.
Overall, they assessed 3,436,957 individuals, with a median age of 45. About 52% were female and 41% were Hispanic, 32% were white, and 12% were Asian or Pacific Islander. About 2.2% tested positive for SARS-CoV-2 via RT-PCR at least once prior to the study period.
About 5% of participants were infected with SARS-CoV-2 during the study period, and of those, about 7% were admitted to the hospital, the authors noted. They added that a higher proportion of those admitted to the hospital were older, men, had comorbidities and prior increased healthcare utilization.
As of August 8, about 1 million participants were fully vaccinated with the Pfizer vaccine. The overall vaccine effectiveness over the course of the study was 73% (95% CI 72-74) and effectiveness against COVID-related hospitalization was 90% (95% CI 89-92).
There were also 8,911 specimens submitted for genomic sequencing beginning on March 4, 2021, and 5,088 (52%) with a sequence determined, the authors noted. Notably, the Delta variant increased in prevalence from 0.6% of sequenced specimens in April 2021 to 87% of sequenced specimens in July 2021.
“Along with other emerging evidence, our results suggest that despite early effectiveness of [Pfizer vaccine] against delta and other variants of concern, effectiveness against infection erodes steadily in the months after receipt of the second dose,” the authors wrote.
Limitations to the data include an inability to establish causation between vaccination and COVID-19 outcomes, as this was an observational study. There was also no data about adherence to public health interventions, and genomic sequencing was “more likely to fail in samples from vaccinated individuals due to lower viral loads,” which could overestimate variant-specific effectiveness, they said.
Molly Walker is deputy managing editor and covers infectious diseases for MedPage Today. She is a 2020 J2 Achievement Award winner for her COVID-19 coverage. Follow
The study was supported by Pfizer.
Tartof disclosed support from Pfizer, Gilead, GlaxoSmithKline, and Genentech.
Other co-authors disclosed various ties to industry, including being employed by Pfizer.