New data suggests an initial Covid-19 vaccine course offers little protection against symptomatic infection caused by the omicron variant. But two new studies reveal that booster shots can help bolster protection against the new variant.

The omicron variant: The 'good,' 'bad,' and 'ugly' scenarios

How effective are vaccines against omicron?

In a British study, which was published Friday and has yet to be peer-reviewed, researchers found that four months after a person received their second dose of the Pfizer-BioNTech Covid-19 vaccine, the shots were about 35% effective at preventing symptomatic Covid-19 caused by omicron. However, that number increased to 75% with a third dose of the vaccine.

Meanwhile, two doses of the AstraZeneca vaccine provided almost no protection against symptomatic Covid-19 caused by omicron months after vaccination, the researchers found. But adding a booster shot of the Pfizer-BioNTech vaccine increased effectiveness to 71%.

The authors of the study added they believe the vaccines are still effective at preventing hospitalization and death from Covid-19 and cautioned that it's too early to know exactly how well the vaccines perform against omicron.

Meanwhile, in a separate study from Israel set to be published in the New England Journal of Medicine, researchers from the Sheba Medical Center and the Israeli Health Ministry looked at the blood samples of 20 people who worked at Sheba and received a booster shot at least a month ago, as well as 20 employees who were five or six months past their second shot but had yet to receive a booster.

The researchers found that those who had received a booster shot were significantly more likely to neutralize omicron than those without. However, booster shots were still less effective against omicron than other variants, including the delta variant.

According to Gili Regev-Yochay, director of the infectious disease epidemiology unit at Sheba, the bad news of the study is that those who received a "second dose of the vaccine five to six months ago don't have any neutralizing ability."

Data from the United States has suggested similar conclusions. On Friday, CDC Director Rochelle Walensky reviewed an early release of the agency's Morbidity and Mortality Weekly Report that looked at 43 omicron cases between Dec. 1 and Dec. 7.

Of those cases, nearly 60% were among those ages 18 to 39, and almost 80% were fully vaccinated, with 14 patients having also received a booster shot, though six of them had gotten their booster less than 14 days before they developed symptoms. Six of the cases involved a person who previously had Covid-19.

In total, there was one hospitalization and no deaths. Virtually all of the cases were symptomatic, with the most common symptoms being cough, fatigue, and congestion or runny nose.

"Although we don't have all the answers on the omicron variant, initial data suggest[s] Covid-19 boosters can help to bolster protection against omicron," Walensky said.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, added that lab data shows two doses of a Covid-19 vaccine aren't as effective against omicron.

"That's the sobering news," Fauci said, but added that boosters of the Pfizer-BioNTech vaccine increase "neutralizing antibody titers by 25-fold" which is "a striking endorsement in the importance of boosting."

Against omicron, getting a booster shot is "essentially equivalent" to a normal two-dose vaccine against other variants, Fauci said.

How transmissible is omicron?

Meanwhile, data released by Britain's Health Security Agency found that a person infected with omicron is about three times as likely to pass the virus on to other members of their household as someone infected with delta.

And someone who is a close contact of a person infected with omicron is about twice as likely to catch the virus as someone who is a close contact of a person infected with delta.

Neil Ferguson, an epidemiologist at Imperial College London, said the modeling work by his team suggests omicron is 25% to 50% more contagious than delta.

And while early data suggests omicron cases may be milder, even if the variant causes severe disease at half the rate of delta, Ferguson said his modeling suggests 5,000 people could be admitted to hospitals each day in Britain at the peak of the omicron wave, which would be a higher number than at any other point in the pandemic.

"It only requires a small drop in protection against severe disease for those very large numbers of infections to translate into levels of hospitalization we can't cope with," Ferguson said. (Mueller, New York Times, 12/12; Lieber, Wall Street Journal, 12/12; Walker, MedPage Today, 12/10)