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Several weeks have passed since Omicron was first identified. What do we know now?

Dr. Anthony Fauci.Anna Moneymaker/Getty

Since the Omicron was first identified in South Africa at the end of November, health officials have been racing to learn more about the new variant as it spreads to dozens of countries and states.

Nearly two years into the COVID-19 pandemic and as the Delta variant remains the dominant variant circulating in the United States, Omicron’s detection prompted a wave of travel restrictions across the globe and uncertainty about how acutely the variant’s spread might be felt.

Experts cautioned that a complete picture of the variant’s transmissibility, whether it causes severe illness, and whether it evades protection brought by vaccines is unclear. But with emerging data, officials are learning more about Omicron each day.


Here’s a look at what we know so far.

What’s the latest?

Federal health officials Wednesday offered an early answer to one of the biggest questions that have emerged since the new variant was detected.

A variant-specific vaccine to protect against Omicron is currently not necessary, the nation’s top infectious disease expert said Wednesday, with data showing boosters developed by Pfizer and Moderna appear to increase protection against the variant.

“Our booster vaccine regimens work against Omicron,” Dr. Anthony Fauci said during a White House COVID-19 briefing. “At this point, there is no need for a variant-specific booster, and so the message remains clear: If you are unvaccinated, get vaccinated, and particularly in the arena of Omicron, if you are fully vaccinated, get your booster shot.”

Public health officials had previously said it was unclear if a new booster would be needed to protect against the new variant as Pfizer and Moderna scrambled to develop the shots just in case. During an address in which he announced a winter COVID-19 strategy, Biden said the administration was working with vaccine manufacturers to develop contingency plans if new boosters are needed.


How well do vaccines protect against Omicron, according to early data?

During the briefing, Fauci described data from the United States and across the world that show how the two-dose mRNA vaccines produced by Pfizer and Moderna fare against the Omicron variant, and highlighted the additional protection boosters provide.

Fauci described data from Pfizer-BioNTech that show an increased ability in vaccine-induced antibodies to protect against the Omicron variant one month after the booster shot compared with three weeks after the second dose. Another study funded by the National Institute of Allergy and Infectious Diseases found that Omicron can evade protection from the vaccines, but people who had two doses of an mRNA vaccine saw increased protection against the variant after they received a booster shot.

Laboratory data from the National Institutes of Health that’s set to be published in a pre-print study next week found that a booster dose of the Moderna vaccine increased antibody responses to Omicron.

In real-world data from South Africa, protection against infection from the Omicron variant dropped to about 30 percent with Pfizer’s two-dose vaccine regimen, but the vaccine was 70 percent effective in preventing against hospitalization.

“Obviously this is significantly down, but there is the maintaining of a degree of protection against hospitalization,” Fauci said.

Data from the United Kingdom found that the Pfizer vaccine’s effectiveness against symptomatic illness is lowered against Omicron, but a booster shot increased protection to 75 percent.

All together, the data indicate the need for people who are eligible for booster shots to get them, Fauci said.

How prevalent is Omicron in the US?

Dr. Rochelle Walensky, the chief of the Centers for Disease Control and Prevention, said the Omicron variant has now been identified in at least 36 states and more than 75 countries.


The CDC estimates that while the Delta variant makes up the overwhelming majority of cases in the US, the Omicron variant constitutes 3 percent of COVID-19 cases in the country. That number is higher in certain places, such as New York and New Jersey, Walensky said, where the CDC projects Omicron makes up 13 percent of all cases.

Officials anticipate that the percentage of Omicron cases in the US will increase, Walensky said.

“In looking at early data on transmissibility of Omicron from other countries, we expect to see the proportion of Omicron cases here in the United States continue to grow in the coming weeks,” Walensky said.

According to the CDC’s projections of the proportions of the variants circulating in each state, Delta makes up 99.53 percent of cases that have been sequenced in Massachusetts.

Early data show Omicron is more transmissible than Delta, Walensky said, adding that the number of Omicron cases is doubling about every two days.

Wednesday, the Boston Public Health Commission said three cases of the Omicron variant were identified in Boston adults, none of whom were fully vaccinated.

The variant was detected in “three Boston young adults over the age of 18,” the commission said in a statement. Further identifying information such as their gender, age, and neighborhood they live in was not provided. All three people had mild cases and did not need to be hospitalized, the commission said.


Massachusetts officials announced on Dec. 5 that the first confirmed Omicron case had been detected in the state in a woman in her 20s who lives in Middlesex County. The woman, who is fully vaccinated, had a case of “mild disease” and did not need to be hospitalized, the Department of Public Health said.

What should people do to protect themselves?

Officials are urging people to get vaccinated and get their booster shots in order to shore up their protection against the virus.

The variant’s transmissibility, coupled with the fact that December holidays are approaching and people are preparing to gather with their loved ones, emphasize the need for people to get vaccinated, boosted, and wear masks in indoor public spaces in communities with significant COVID transmission, Walensky said.

About 90 percent of all counties in the United States have been deemed areas of “high” or “substantial” transmission by the CDC. Currently, every county in Massachusetts has “high” COVID transmission, according to the CDC.

The emergence of the Omicron variant comes as COVID-19 cases and hospitalizations are increasing in Massachusetts in what experts fear is the beginning of a winter surge, and the United States is seeing a high volume of cases. More than 200,000 COVID-19 cases were reported to the CDC on Monday, and the current seven-day daily average of cases is 117,891, Walensky said.

More than 200 million people, or about 61 percent of the US population, are fully vaccinated, and more than 55 million people, or about 27 percent of the population, have received booster shots, according to CDC data.


Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1.