The Omicron Subvariant Is More Contagious, But Vaccinated People Are Less Likely to Spread It, Study Finds

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  • Danish scientists found the BA.2 subvariant, which has rapidly become dominant in Denmark, spread more easily across all groups regardless of sex, age, household size and vaccination status.
  • However, only unvaccinated people demonstrated higher transmissibility of BA.2 compared to the BA.1 omicron strain, which could be due to their higher viral load.
  • The probability for spreading within a household was 39% for BA.2 versus 29% for BA.1, the study found.

The omicron BA.2 subvariant is inherently more contagious and better at evading vaccines than any other Covid strain, but vaccinated people don't transmit it as easily as the unvaccinated, according to a Danish study published Sunday.

The new subvariant, which has rapidly become dominant in Denmark, spread more easily across all groups regardless of sex, age, household size and vaccination status, the study found.

The probability for spreading within a household was 39% for BA.2 versus 29% for BA.1, the original omicron strain that was dominant worldwide as of Jan. 19, according to the World Health Organization.

The study, led by a team of scientists affiliated with the University of Copenhagen and the Danish Health Ministry among other institutions, has not yet been submitted for peer review. Researchers have been publishing their findings before they are examined by other experts in the field due to the urgent nature of the pandemic.

BA.2 is more contagious than the original BA.1 strain among both vaccinated and unvaccinated people, but the relative increase in susceptibility to infection was significantly greater in vaccinated individuals than unvaccinated individuals. That indicates the subvariant is even better at escaping vaccine protection than BA.1, which was already significantly more contagious than any other Covid variant, according to the study.

Transmission rates among unvaccinated people were higher with BA.2 compared to BA.1, indicating unvaccinated people were carrying a higher viral load with BA.2. Although fully vaccinated people are more likely to catch BA.2 than the previous strain, they are less likely to spread it to others, researchers found.

People who received a booster were even less likely to transmit the virus than people who were fully vaccinated.

"This indicates that after a breakthrough infection, vaccination protects against further transmission, and more so for BA.2 than BA.1," the scientists found.

The study also noted that the higher susceptibility to infection and greater transmissibility of BA.2 will likely result in more extensive spread of the virus among unvaccinated kids in schools and day care.

It is reassuring that Covid BA.2 infections are generally milder than delta variant infections, the scientists said, and that vaccines help protect against severe illness and hospitalizations.

"The combination of high incidence of a relative innocuous subvariant has raised optimism," the scientists wrote, while noting the importance of keeping a close eye on BA.2.

More than half of the states in the U.S. have detected BA.2, with a total of 194 confirmed cases nationwide so far, according to a global database of Covid variants. The Centers for Disease Control and Prevention, in a statement Friday, said BA.2 is currently circulating at a very low level in the U.S.

"Currently there is no evidence that the BA.2 lineage is more severe than the BA.1 lineage," CDC spokesperson Kristen Nordlund said.

Denmark, a country of 5.8 million people, is reporting an average of about 46,000 new cases per day, according to data compiled by Johns Hopkins University, up 18% over the past week and nearly double the level of two weeks ago. The Scandinavian nation reported 80 new hospitalizations on Monday for a total of more than 1,000 people currently admitted with Covid.

Denmark is reporting an average of roughly 19 Covid deaths per day, which is on the rise but well below its pandemic high of about 36 daily deaths recorded last winter, according to the Hopkins data.

Troels Lillebaek, chairman of Denmark's Covid variant surveillance committee, said the health-care system in his country might be able to handle the hospitalizations while nations with lower vaccination rates could face a rougher road ahead.

"If you are in a community or living in a country where you have a low vaccination rate, then you will have for sure more admissions to hospital and more severe cases and then more going to ICU," Lillebaek said.

In Denmark, more than 80% of the population are fully vaccinated and more than 60% have received a booster dose. In the U.S., 63% of the total population are vaccinated, and about 41% of the vaccinated have received a booster, according to the CDC.

The World Health Organization hasn't yet labeled BA.2 a separate variant of concern from omicron. However, WHO officials have warned that new variants will almost certainly emerge as omicron spreads at an unprecedented rate around the world. Maria Van Kerkhove, the WHO's Covid-19 technical lead, said last week that the next variant will be more transmissible, but it's an open question whether it will be more severe.

"The next variant of concern will be more fit, and what we mean by that is it will be more transmissible because it will have to overtake what is currently circulating," Van Kerkhove said. "The big question is whether or not future variants will be more or less severe."

The CEOs of Pfizer and Moderna are also worried that a new variant could emerge as immunity from vaccines wanes off over time. Pfizer is conducting a clinical trial of an omicron vaccine in people aged 18 to 55. The company expects to have the shot ready by March. Meanwhile, Moderna has launched a clinical trial of an omicron-specific booster dose in adults over the age of 18.

Studies from around the world — including the U.S., the U.K. and South Africa among others — have found that omicron generally does not make people as sick as the previous delta variant. However, omicron is spreading so quickly that it is putting strain on already overburdened hospitals.

The WHO and White House chief medical advisor Dr. Anthony Fauci have said eradicating Covid is unlikely. However, the WHO's director for Europe, Hans Kluge, said last week that omicron "offers plausible hope for stabilization and normalization."

Public health leaders and scientists around the world hope vaccination and mass exposure to omicron will create so much immunity in populations that fewer people will be susceptible to the virus, which could make it less disruptive to society.

However, Fauci has said there's no guarantee that the omicron variant will help end the pandemic. "I would hope that that's the case. But that would only be the case if we don't get another variant that eludes the immune response of the prior variant," he told the World Economic Forum earlier this month.

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