According to a study by researchers at the University of Oxford, the effectiveness of the Pfizer and AstraZeneca coronavirus vaccines against the Omicron variant has decreased significantly.
The results suggest that the Omicron variant could trigger a new wave of coronavirus infections, the researchers said, although there’s no evidence it would lead to an increase in serious illness or death.
“Our findings demonstrate that the effectiveness of vaccines against symptomatic diseases with the Omicron variant is significantly lower than with the Delta variant,” the researchers wrote. These findings are consistent with other recently published studies on the effectiveness of vaccines against the Omicron variant.
The Oxford researchers used blood samples collected from people who had received two doses of the AstraZeneca or Pfizer Covid-19 vaccines. For the Pfizer vaccine, efficacy against Omicron was lower than against Delta, except for two to nine weeks after the second dose. Among those who received two doses of the AstraZeneca vaccine, “there was no protective effect of vaccination against symptomatic disease with Omicron from 15 weeks after the second dose.”
These results may be due in part to differences in the populations that received the vaccines. The AstraZeneca vaccine was used early on in the UK’s vaccination programme, including in nursing homes and in older and more at-risk populations. The high level of efficacy of the Pfizer vaccine against Omicron at two to nine weeks after the second dose is likely in recently vaccinated young adults and teenagers.
The research is released as a preprint, meaning it has not been peer reviewed or published.
The researchers noted that they were unable to determine whether vaccines protect against serious illness due to the small number of Omicron cases and the delay between infection and serious illness. “It will take some time to estimate effectiveness against severe disease with Omicron, but based on experience with other variants, this is likely to be significantly higher than estimates against symptomatic disease,” they wrote.
“These data are important, but only part of the picture,” Matthew Snape, co-author of the paper and professor of pediatrics and vaccinology at the University of Oxford, said in a statement. “They don’t look at neutralizing antibodies until after the second dose, but don’t tell us anything about cellular immunity, and this will also be tested with stored samples once the tests are available.”
“Importantly, we have not yet assessed the impact of a ‘third dose’ booster, which we know significantly increases antibody concentrations, and is likely to lead to improved potency against the Omicron variant.”
