First large Omicron study detects two doses of Pfizer vaccine Cuts risk of hospitalization by 70%

While the study provides important clues as to how the vaccine works against Omicron, it’s difficult to draw broad conclusions from South Africa, which has a much younger population than Europe and the US. and also have other immune admixture with high degree. of previous infection but vaccination rates are relatively low. For conclusions that may be more relevant to the US, health authorities will scrutinize the UK, whose demographic profile and vaccination rates are more similar to the US, and where the variation is already well established.

“Glad to see this result and we still have the vaccine [against hospital admission] Glenda Gray, president and chief executive officer of the South African Medical Research Council, which collaborated with Discovery, said.

The study examined 211,610 positive Covid-19 test results in adults reported since early September. It used that data to compare the effectiveness of the vaccine during September and October, when Delta dominated, with a three-week period from November 15 to December 7, when Omicron took hold. Discovery Health covers approximately 3.7 million people in South Africa.

The study – the largest to provide clues to how vaccines work against Omicron in the real world – shows that although the new strain of the virus can easily infect fully vaccinated people, it are still much less likely to cause serious illness. . The study has yet to be published or peer-reviewed in a scientific journal, and scientists who were not involved in the study said the conclusions could change as more data becomes available.

Scientists identify the Omicron . variant for the first time in South Africa about three weeks ago and has fueled a dramatic increase in cases there. On Friday, scientists estimated South Africa’s R-number of Omicrons – a measure of how many people on average go on to infect – to stand at 2.5, higher than any previous variation.

Daily new cases averaged 20,488 cases in the week ending December 13, nearly double from the previous week. On Monday, in a sign that a large number of infections are being missed, health authorities said 31% of tests were positive.

The findings build on earlier research in the lab from different groups around the world that tested how well the blood of vaccinated people neutralized the Omicron variant. Those studies found that the antibodies in the blood of people who had received two doses of the vaccine were much weaker against Omicron than previous strains.

Last week, Pfizer executives predicted that vaccines would be better at fighting severe disease because immune cells that fight the virus once it’s in hold can still recognize most parts of the virus. mutant protein of Omicron, which the virus uses to enter cells.

Neutralizing antibodies act as the body’s first line of defense, aiming to prevent infection by preventing viruses from entering cells. Other parts of the immune system, such as T cells, work to prevent serious illness after an infection.

The Discovery study also found that protection against infection from Omicrons seems to wane over time in vaccinated people. People who received a second dose in the two to four weeks before the Omicron period were 56% protected from infection with the new strain of bacteria. That protection drops to 25% for people who received a second dose three to four months earlier. In the study, the infection was associated with a positive PCR test result, so likely reflecting symptomatic disease, the researchers said.

Protection against serious illness appears to decline with age, but the researchers warn that the data are uncertain and could be complicated by a greater weakening effect in older groups, who were vaccinated earlier. The researchers also found that Omicrons eroded the protective effects of previous infections.

The study could not test the actual effect of the third shot because South Africa only recently approved the booster and has not yet begun to roll it out. But it’s possible that a booster would enhance protection against infection, the researchers say. Pfizer and BioNTech said last week third dose restores antibodies to the point where they were able to block the Omicron variant in lab tests.

“Vaccines are designed to protect against hospitalization and death,” said Shirley Collie, head of health analytics at Discovery Health. “These are breakthrough infections that we expect to see. This is something a promotion strategy will minimize. “

Separately, the study found that, adjusting for various factors, including age, adults infected with the Omicron variant were 29% less likely to need hospitalization than during the first episode of the disease. country, which is dominated by a strain known as D614G. It also found that, among those hospitalized, the illness appeared to be less severe, with 5% of hospitalized patients requiring intensive care, compared with 22% during the Delta episode.

The researchers said they were unable to determine whether Omicron was inherently less virulent than previous strains or the lower hospitalization rate was due to a high level of immunity in the population from previous infection or vaccination.

It also found that children were 20 percent more likely to be hospitalized with the virus than in the first wave, but the researchers say the figure may only reflect higher rates of infection among children. Children are admitted to non-Covid care, as hospitals regularly check all admissions.

—Alexandra Wexler Snow and Aaisha Dadi Patel contributed to this article.

Write letter for Denise Roland at Denise.Roland@wsj.com