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The Omicron variant of SARS-CoV-2 is much more effective at infecting people vaccinated against the COVID-19 disease it causes, a review of research on the subject suggests.
Research also indicates vaccination provides less protection against serious disease upon Omicron infection.
The Omicron variant, first detected in South Africa in November, now constitutes the dominant strain in the United States, according to estimates by the Centers for Disease Control and Prevention (CDC). It comprised about 31-45 percent of infections in the week ending Dec. 18, according to the latest CDC variant surveillance data. In the week ending Jan. 1, it was already 93-97 percent, the agency projected.
Omicron appears to spread more easily, but with less severe symptoms.
A recent Danish study found that a fully vaccinated individual in an Omicron-infected household was about as susceptible to get infected as an unvaccinated individual. Boosted individuals were about half as susceptible. That’s substantially worse than what the vaccines could do against Delta, where they provided a 57 percent drop in susceptibility even without a booster.
The study indicated that among the unvaccinated, Omicron spreads only somewhat more easily than Delta. It was among the vaccinated that the variant made the largest headway, leading the researchers to suggest the virus has evolved in a way to dodge the vaccine-induced immunity.
When it comes to spreading the virus for those vaccinated or unvaccinated, Omicron was about the same as Delta. Unvaccinated have about 40 percent higher odds of infecting somebody than the vaccinated and about twice the odds of the boosted, the Danish study found by looking at the spread of the virus in over 12,000 households.
Half a year after the needle, the Pfizer vaccine’s effectiveness at preventing severe disease caused by Omicron drops to somewhere between 33 and 92 percent, a recent meta-analysis estimated. The Moderna vaccine fared better, with 42-96 percent effectiveness.
Researchers with the Houston Methodist Hospital and other institutions did a sweep in November and December for symptomatic Omicron infections in Houston. Of the 862 identified cases, half (430) were vaccinated and 85 had had a booster as well.
The percentage of vaccinated patients was much higher for Omicron than for prior dominant strains, according to researchers. The percentage jumped from 3.2 percent for Alpha (the initial virus variant) to 24.1 percent for Delta to 49.9 percent for Omicron.
Omicron patients were “significantly younger” than Alpha or Delta patients, “significantly fewer” required hospitalization, and their median length of hospital stay was “significantly shorter,” the researchers found.
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