Study Confirms Benefits of COVID-19 Vaccines, Shows Effectiveness Wanes Over Time

A long-term study of the effectiveness of COVID-19 vaccines confirmed protection against SARS-CoV-2 infection, hospitalization and death, but those benefits waned markedly over time.

The systematic review, published in The Lancet Respiratory Medicine, analyzed 68 studies that reported vaccine effectiveness immediately and at least 112 days after a primary vaccine series and at least 84 days after a booster dose.

“Our analyses indicate that vaccine effectiveness generally decreases over time against SARS-CoV-2 infections, hospitalisations, and mortality,” the authors concluded. “The baseline vaccine effectiveness levels for the omicron variant were notably lower than for other variants. Therefore, other preventive measures (eg, face-mask wearing and physical distancing) might be necessary to manage the pandemic in the long term.”

The investigators searched the Excerpta Medica Database and the US National Institutes of Health iSearch COVID-19 portfolio and manually searched COVID-19 sources for studies published between Jan. 1, 2020, and Dec. 1, 2022. The research included the four COVID-19 vaccines licensed in Canada from Pizer-BioNTech, Moderna, Oxford-AstraZeneca, and Janssen.

The study found that overall vaccine effectiveness of the primary series against infection was 83% at 14 to 42 days after vaccination and fell to 62% by 112 to 139 days and 47% by 280 days. Initial effectiveness against hospitalization was 92%, and fell to 79% at 224 to 251 days. Against death, effectiveness was 91%, and fell to 86% at 168 to 195 days.

Effectiveness of booster doses was 70% against infection and 89% against hospitalization, and fell to 43% and 71% respectively at 112 days or later.

Vaccines were less effective against Omicron variants–61% against infection and 71% against hospitalization at 14 to 42 days, falling to 21% and 52% respectively at 168 to 195 days.

The World Health Organization defines adequate effectiveness as at least 70% against infection and at least 90% against hospitalization and death.

“We were surprised that the vaccines didn't hold for very long and how much the Omicron variant really reduces effectiveness,” Simon Bacon, PhD, of the Department of Health, Kinesiology and Applied Physiology at Concordia University, Montreal, told Contagion.

The study was funded by the Canadian Institutes of Health Research and the Public Health Agency of Canada and could help health policymakers determine strategies for addressing the virus.

“Vaccines work, though the effect does drop within 3-4 months, this is true for both the primary series and boosters, so continuing to administer booster doses is going to be important,” Bacon said.

The authors advocated for mitigation measures such as wearing masks, physical distancing and quarantining to reduce COVID-19 spread and said further research is needed to assess the effectiveness of mitigation measures among vaccinated people.

All vaccine classes saw similar drops in effectiveness over time, but adenovirus vaccines had lower overall effectiveness against infection than mRNA vaccines. Against hospitalization and death, both classes of vaccines showed similar results.

At 14 to 42, effectiveness against infection by any variant was 87% for mRNA vaccines and 69% for adenovirus vaccines, dropping to 66% and 56% respectively at 112-139 days. Effectiveness against hospitalizations was dropped from 93% to 89% during that time for mRNA vaccines and from90% to 89% for adenovirus vaccines.

Bacon said research will continue to track the effectiveness of further booster doses and analyze various combinations of vaccines and hybrid immunity.

“If we have a major different variant appear we'd need to see how the vaccines hold up against that,” he said.