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In a study published in JAMA Network Open today, scientists report on how effective the original vaccine and booster shots are against the COVID-19 Delta and Omicron variants.

Researchers in Ontario analyzed data from more than 134,000 people, including those who tested positive for Delta and Omicron infections during December 2021. They found that people who were fully vaccinated (with two doses of an mRNA vaccine, from either Moderna or Pfizer-BioNTech) experienced a decline in vaccine effectiveness against both Delta and Omicron infections, but the drop was greater against Omicron than against Delta. Among the vaccinated, the shots’ effectiveness declined from 36% up to two months after the second dose of the primary series, to 1% up to four months later (or six months after the second dose).
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Booster doses helped restore some of the vaccine’s effectiveness, bringing it back up to 61% against Omicron beginning a week after people received the booster shot.

“The bottom line message is that against Omicron, you really need three doses for optimal protection against severe outcomes,” says Dr. Jeff Kwong, senior scientist at ICES (a not-for-profit research institute) and the study’s senior author. “Two doses was good enough against Delta, but since last December, when Omicron took over, two doses does not provide quite enough protection.”

Read More: I’ve Had COVID-19. Do I Still Need the Omicron Booster?

The study did not explore how long that protection lasts after the third shot, or the first booster dose. U.S. health officials now recommend that people receive another booster dose, the first one that specifically targets Omicron. The booster contains genetic sequences of Omicron BA.4/5, which now causes nearly all new infections of COVID-19. Based on the data from his study, which showed waning of protection after the primary vaccination series, Kwong anticipates that the same will happen after the first booster. If antibodies wane, then people are less protected from getting infected with the virus.

On the plus side, Kwong’s study confirmed previous data showing that vaccinated people who also received a first booster dose remain protected from getting seriously ill with COVID-19, even if they are infected with Omicron; vaccine effectiveness against severe disease was about 95% a week or more after the third dose. The new Omicron-based booster, which targets both the original and Omicron BA.4/5 variants, “is a good move for sure,” says Kwong, to improve people’s protection from getting infected. But, he says, “my worry is that there could be yet another variant that emerges with other mutations. And this Omicron booster may or may not help against that.”

Read More: Should You Mix and Match Omicron Boosters? Here’s What to Know

The study data are a good reminder that vaccines can’t provide perfect protection, particularly against getting infected, Kwong says. For that, other measures may be more effective, including wearing masks and avoiding crowded indoor gatherings with poor ventilation. “We need other measures to better protect ourselves, and masking is one that doesn’t care what variant is circulating,” he says. “It’s unfortunate that masks have become so politicized, but the more people are wearing masks, the more protected everybody is.”

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