Two doses of either the Pfizer-BioNTech or the Moderna Covid-19 vaccines provide minimal protection against symptomatic illness caused by the omicron variant of the coronavirus, but a booster shot was able to restore protection, new research finds.
The study, published Wednesday in the New England Journal of Medicine, found that the Pfizer vaccine’s effectiveness against symptomatic disease plummeted to about 8 percent from about 65 percent six months after the second dose of a primary vaccination series.
The effectiveness of the first two doses of the Moderna vaccine was similarly reduced, falling to about 15 percent from about 71 percent over the same period.
A booster shot of either the Pfizer or the Moderna vaccines increased the protection back to levels seen after two doses, before it starts to wane again after about two months, according to the researchers.
The study, funded by the U.K. Health Security Agency, adds to growing evidence that two doses of the existing Covid vaccines appear to offer little protection against mild illness from the omicron variant, although experts note that the shots still provide strong protection against severe disease and death.
The findings underscore the importance of the booster shot, the authors wrote, adding that third doses provide a “rapid and substantial” increase in protection against both mild and severe illness.