This is how effective the original Omicron vaccines are

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

Researchers in Ontario analyzed data from more than 134,000 people, including those who tested positive for Delta and Omicron infections in December 2021. They found that people who were fully vaccinated (with two doses of Moderna’s mRNA vaccine or Pfizer-BioNTech) experienced a decline in vaccine effectiveness against both Delta and Omicron infections, but the decline was greater against Omicron than against Delta. Among those vaccinated, vaccine effectiveness declined from 36% by two months after the second dose of the primary series, to 1% by four months later (or six months after the second dose).

Booster doses helped restore some of the vaccine’s effectiveness, bringing it back up to 61% against Omicron starting a week after people received the booster shot.

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

Read more: I had COVID-19. Do I still need the Omicron amplifier?

The study did not examine how long this protection lasted after the third injection or the first booster dose. US health officials are now recommending that people get another booster dose, the first specifically targeted at Omicron. The booster contains genetic sequences of Omicron BA.4/5, which now causes almost all new COVID-19 infections. Based on the data from his study showing a waning of protection after the primary series of vaccinations, Kwong expects the same to happen after the first booster. If antibodies decrease, people are less protected from contracting the virus.

On the plus side, Kwong’s study confirmed previous data showing that vaccinated people who also received a first booster dose remained protected from becoming seriously ill with COVID-19 even if infected with Omicron; the vaccine’s effectiveness against severe disease is about 95% a week or more after the third dose. The new Booster based on Omicron, which targets both the original variant and the Omicron BA.4/5, “is certainly a good move,” says Kwong, to improve people’s defenses against infection. But, he says, “my concern is that there may be another variant that appears 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 you need to know

The study data is a good reminder that vaccines cannot provide perfect protection, especially against infection, says Kwong. For this, 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 something that doesn’t care what variant is circulating,” he says. “It’s unfortunate that masks have become so politicized, but the more people wear masks, the safer everyone is.”

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