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SciCheck’s COVID-19/Vaccination Project

Early Data on Omicron

This article is available in both English and Español

As many Americans and others around the globe prepare for holiday travel and get-togethers, the omicron variant continues to be a concern. While the available data on transmission, vaccine effectiveness and severity of disease are still preliminary, here’s what we’ve learned so far.


Omicron has quickly taken over as the dominant variant in the United States. On Dec. 1, the country had its first confirmed case, and by the week ending Dec. 25, omicron accounted for an estimated 59% of new cases, according to the Centers for Disease Control and Prevention. (See “Variant Proportions.”)

Overall, the seven-day average of new cases in the U.S., due to any variant, has gone up, numbering about 80,000 on Nov. 29 and 135,000 on Dec. 19.

The World Health Organization said on Dec. 17 that omicron has been identified in 89 countries and that there is “consistent evidence” the variant “has a substantial growth advantage” over delta. “It is spreading significantly faster than the Delta variant in countries with documented community transmission, with a doubling time between 1.5–3 days,” the WHO said in a technical brief. “Omicron is spreading rapidly in countries with high levels of population immunity and it remains uncertain to what extent the observed rapid growth rate can be attributed to immune evasion, intrinsic increased transmissibility or a combination of both. However, given current available data, it is likely that Omicron will outpace Delta where community transmission occurs.”

As we explained in an earlier story on omicron, scientists were concerned about how easily the variant could be transmitted because several of the mutations present in the genome, including some at the virus’s furin cleavage site, have been linked to increased transmissibility. That site is a spot on the virus’s spike protein that’s cut by the enzyme furin to activate the spike and prepare the virus for entering cells.

Hong Kong University researchers released information from an early lab study, not peer-reviewed, on Dec. 15 that found the omicron variant multiplies in respiratory-tract tissue 70 times faster than delta or the original SARS-CoV-2 over the first 24 hours, “which may explain why Omicron may transmit faster between humans than previous variant,” a press release on the findings said

Researchers with the Ragon Institute in Massachusetts and others posted a not-yet-peer-reviewed study on Dec. 14 that found an omicron pseudovirus tested in the lab was “more infectious” than delta or the original version of the virus due to a more efficient ability to attach to the ACE2, the human receptor that the virus uses to gain entry into cells. The study found omicron was twice as infectious as delta.

There is also some evidence that omicron could have a shorter incubation period — the time between exposure to the virus and the start of symptoms — than previous variants. For instance, a case study on omicron’s spread at a company Christmas party in Norway, where 74% of attendees later had confirmed or probable omicron cases, showed the incubation period ranged from zero to eight days, but the median was three. “Assuming attendees were infected at the party, we observed a median incubation period of 3 days, which is short compared with previous reports for Delta and other previously circulating non-Delta SARS-CoV-2 (4.3 and 5.0 days, respectively),” according to an article on the outbreak in the journal Eurosurveillance.

As with many aspects of the variant, more research is needed on the incubation time. But if it is shorter, that’s not good news in terms of the virus’s ability to quickly move from person to person. As The Atlantic explained in a Dec. 20 article: “These trimmed-down cook times are thought to play a major part in helping coronavirus variants spread: In all likelihood, the shorter the incubation period, the faster someone becomes contagious—and the quicker an outbreak spreads.”

Vaccine Effectiveness and Reinfection

So far, lab and real-world studies, not yet peer-reviewed, indicate COVID-19 vaccines are less effective against omicron — but booster doses provide increased protection and the primary doses are likely to retain some protection against severe illness. But, again, this is preliminary data, based on estimates and often small sample sizes, and more research is needed.

Preliminary lab results from South African researchers and others showed a 41-fold drop in neutralizing ability

Travelers at Los Angeles International Airport on Dec. 21, amid a surge in omicron variant cases. Photo by Mario Tama/Getty Images.

against omicron in 12 people vaccinated with two doses of the Pfizer vaccine. It also showed that five of six people who had hybrid immunity — two doses of the vaccine plus a prior infection — had “relatively high neutralization titers” against omicron.

The Imperial College London COVID-19 response team estimated that the risk of reinfection with the omicron variant is 5.4 times the reinfection risk of the delta variant, based on PCR-confirmed cases in England between late November and Dec. 11. The study, which hasn’t been peer-reviewed, estimated vaccine effectiveness against symptomatic infection after two doses of the Pfizer/BioNTech or AstraZeneca vaccine would be between 0% and 20%. The study estimated 55% to 80% protection against symptomatic infection after a third booster dose.

A non-peer-reviewed study from South Africa’s largest health insurance company, based on more than 211,000 PCR-positive tests in the first three weeks of omicron’s spread, found a higher risk of reinfection compared with other variants. It also said vaccine effectiveness against severe disease requiring hospitalizations during the omicron surge was 70% with two doses of the Pfizer/BioNTech vaccine, and effectiveness against infection was 33%.

For comparison, there was an estimated 80% effectiveness against infection for the mid-2020 wave.

The press release noted that the preliminary information “may change as the wave progresses,” and that the data may be affected by the high level of antibodies in the country’s population overall. In other words, while South Africa has a relatively low rate of vaccination, it has a high rate of prior infection. So most people had some level of immunity against the virus.

“There are still limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for Omicron,” the WHO said on Dec. 17. It advises caution in interpreting preliminary information from South Africa and England due to possible “selection bias” in the studies and “relatively small numbers.”

Pfizer announced on Dec. 8 that preliminary lab tests showed the blood samples from those getting the two-dose series alone experienced “significantly reduced neutralization titers,” but the third booster dose prompted a 25-fold increase in neutralizing antibodies against omicron. The level of neutralizing antibodies after the booster was similar to the level against the original, non-mutated strain after two doses.

Pfizer also said “two doses may still induce protection against severe disease” from the omicron variant, because vaccine-induced T cells can still largely recognize the omicron variant. This suggests that vaccinated individuals would be able to mount a strong T cell response and be unlikely to experience severe disease if infected.

The lab tests evaluated sera from the blood of 19 people who had received their second dose one month earlier and 20 people who had received their booster dose one month earlier.

Moderna said on Dec. 20 that lab results from 20 booster recipients showed the authorized booster dose of its vaccine prompted a 37-fold increase in the level of neutralizing antibodies against omicron, compared with the level before the booster dose. Moderna’s CEO, Stéphane Bancel, called the results “reassuring.”

Both Pfizer and Moderna said they are continuing to test an omicron-specific vaccine in case it is needed.

The Moderna lab tests evaluated blood samples from people 29 days after receiving a booster dose. The authorized booster is a half-dose of the initial series. Moderna also tested sera from people receiving a booster that contained the full mRNA dose as the initial vaccine shots; that increased neutralizing antibody levels 83-fold. And it tested “multivalent” boosters, which address mutations of the beta and delta variants, finding similar increases in neutralizing antibodies as the authorized doses.

Johnson & Johnson has not yet released information on lab tests of its vaccine against omicron.

The Ragon Institute researchers also studied the ability of sera from more than 200 vaccinated individuals to neutralize omicron pseudoviruses. They found the samples from those vaccinated with the standard series of shots from Moderna, Pfizer and Johnson & Johnson vaccines didn’t neutralize omicron, but those who had received a booster of an mRNA vaccine showed “potent neutralization” similar to the level seen with the original version of the virus.

Severity of Disease

Early anecdotal reports and a few preliminary studies have suggested omicron could cause less severe disease than other variants, but the virulence is still being studied.

There also have been confirmed deaths due to omicron.

The WHO said on Dec. 17 that more data are needed to assess the severity of disease, but regardless, the rise in cases could stress the health care system. “Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems,” said Tedros Adhanom Ghebreyesus, the WHO’s director-general.

The Hong Kong University study found that while the omicron variant replicated quickly in respiratory-tract tissue, it multiplied more than 10 times less efficiently in human lung tissue than the original coronavirus, “which may suggest lower severity of disease,” the press release on the study said.

But one of the researchers cautioned that severity of disease also depends on how the human immune system responds to infection — not only how well the virus replicates in tissue. “It is also noted that, by infecting many more people, a very infectious virus may cause more severe disease and death even though the virus itself may be less pathogenic,” Dr. Michael Chan Chi-wai, of the Centre for Immunology and Infection, Hong Kong Science and Technology Park, said.

Imperial College London initially found no evidence that omicron produced less severe cases than delta, but in a subsequent report on cases identified from Dec. 1 to 14, researchers found evidence of a reduced risk of hospitalization, compared with delta, of 20% to 25% for any hospital visit and 40% to 45% for hospitalizations of one day or more, or those recorded as “admitted.”

Through Dec. 16, there have been seven deaths associated with omicron in the United Kingdom and 85 hospitalizations, according to the UK Health Security Agency.

Early national data from Scotland, showing 23,840 cases and 15 hospitalizations, suggest a two-thirds reduction in the risk of hospitalization from omicron compared with delta. That non-peer-reviewed data was released Dec. 22.

The South Africa study by the country’s largest health insurer found a “significantly lower” risk of severe disease.

It found a 29% lower risk of hospitalization for omicron compared with a wave of infections in mid-2020, with the data adjusted for vaccination status. Among the positive cases, 41% were adults who had received two doses of the Pfizer/BioNTech vaccine.

The CDC released early information on 43 cases attributed to omicron in the U.S. from Dec. 1 to 8, saying 34 of those individuals were fully vaccinated, including 14 who had gotten a booster dose. Six people had a prior SARS-CoV-2 infection. Symptoms were generally mild – cough, fatigue, congestion, runny nose – though one vaccinated person spent two days in the hospital. There were no deaths in those cases as of Dec. 10. But the CDC said that “as with all variants, a lag exists between infection and more severe outcomes.”

On Dec. 20, a Houston-area man infected with the omicron variant died, possibly the first death due to the variant in the United States. Harris County Public Health said in a press release that he was unvaccinated, between the ages of 50 and 60, and previously infected with SARS-CoV-2. He also had underlying health conditions, putting him at higher risk of a severe case.

For more on omicron’s emergence and initial reports on the variant, see our Dec. 3 story “Q&A on the Omicron Variant.”

Update, Dec. 28: We updated this story with the CDC’s latest estimate for omicron’s prevalence in the United States. For the week ending Dec. 25, the CDC estimates the omicron variant accounted for about 59% of U.S. cases.

In issuing its latest estimate, the agency also significantly revised its estimate for the previous week ending Dec. 18. The CDC’s initial estimate was 73%, which was included in this story. However, the CDC now estimates that the prevalence of omicron for that week was 22.5%. 

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over our editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.