A fourth dose of the COVID vaccine doesn’t do much to defend young and healthy people against catching the virus, but it does give “moderate” protection from developing symptomatic illness, new Israeli research indicates.
The findings, based on monitoring of Israeli health workers during the Omicron wave, were peer-reviewed and published in The New England Journal of Medicine.
The research will likely impact medical discussions internationally on whether there is a benefit to rolling out fourth vaccine doses among the general population.
In the White House, the case for fourth doses “is being very carefully monitored in real time,” lead epidemiologist Dr. Anthony Fauci said in a press briefing last month.
There is also speculation that the US Food and Drug Administration could soon authorize fourth doses for the general population.
Prof. Gili Regev-Yochay, an infectious disease specialist at Sheba Medical Center outside Tel Aviv, said the research showed the extra dose did not have a major impact on virus spread. She led the study at her institution, which sampled some of its health workers who were under the age of retirement.
“In terms of the effectiveness of the fourth vaccine – both Pfizer and Moderna – against infection, we found that the infection rates among vaccinated individuals were only slightly lower than those in the control group,” Regev-Yochay said.
People who received a fourth shot of Pfizer vaccine were 30 percent less likely to be infected than people who got three doses. Among the Moderna recipients, people were 18% less likely to get infected after a fourth shot.
But Regev-Yochay said that there is a stronger effect against symptomatic illness. Pfizer recipients were 43% less likely to develop symptomatic illness after a fourth shot, and among Moderna participants the figure was 31%.
She emphasized that her research was conducted on health workers under the retirement age, and therefore doesn’t give a picture of the fourth dose’s effectiveness for the elderly, which its main target in Israel and in some other countries, including the UK.
The study’s authors wrote that their research indicates the advantages of three doses designed against the original strain of coronavirus had hit a ceiling in terms of immune response, with additional boosters only restoring waned immunity, rather than taking it to new heights.
They highlighted “low vaccine efficacy against infections in health care workers, as well as relatively high viral loads,” and said these findings suggest that “a fourth vaccination of healthy young health care workers may have only marginal benefits.”
They also stated that the research gives reassurance on the safety of fourth doses, writing: “Our data provide evidence that a fourth dose of mRNA vaccine is immunogenic, safe, and somewhat efficacious.”
The new research is based on 600 participants, 270 of whom received either a fourth dose of the Pfizer vaccine or Moderna vaccine.
Regev-Yochay said the results shouldn’t dampen enthusiasm for third vaccine doses, which have been shown to have a dramatic effect on protection.
“It should be emphasized that the third dose is extremely important for anyone who has not yet contracted COVID-19,” she said.
The new research is part of an ongoing effort to monitor the health of a large sample of Sheba Medical Center employees through the pandemic.
“This study is added to a series of studies led by Sheba with the aim of providing a scientific basis for managing a pandemic that has wreaked havoc around the globe,” stated Regev-Yochay.
“Thanks to the cohort at Sheba and the plethora of data we have accumulated since the beginning of the pandemic, we continue to lead international studies, which shed light on the behavior of the virus and the effectiveness of vaccines and serve decision makers in determining health policy in Israel and around the world,” she said.
AFP contributed to this report.