Two doses of a COVID-19 vaccine were significantly less protective against Omicron compared with Delta, but a booster shot restored immunity back to high levels, according to real-world data from the U.K.
A two-dose series with the Pfizer/BioNTech vaccine provided just under 40% protection against symptomatic infection with the Omicron variant about 25 weeks after the second dose compared with around 60% protection against Delta, a found.
"These early estimates suggest that vaccine effectiveness against symptomatic disease with the Omicron variant is significantly lower than compared to the Delta variant," the agency noted in the report. However, "moderate to high" vaccine effectiveness was observed in the early period after a booster shot, they added.
A Pfizer/BioNTech booster increased vaccine effectiveness to 76%, the agency found. Among people who received the AstraZeneca series for their initial immunization (which offered almost no protection against Omicron), vaccine effectiveness jumped to 71% after a Pfizer/BioNTech booster.
The U.K.'s research compared vaccine effectiveness against Omicron versus Delta, and included 581 people who were infected with the new strain and more than 56,000 infected with Delta from the end of November to December 6.
In a separate analysis, the U.K. data showed that the rate of reinfection with Omicron was also much higher than reinfection with Delta. Of 329 individuals infected with Omicron, 7% had a previous infection, compared with 0.4% of the approximately 85,000 people infected with Delta.
After adjusting for age, public health region, and collection pillar, the risk ratio of reinfection for Omicron was 5.2 (95% CI 3.4-7.6).
The report also found a 20- to 40-fold reduction in neutralizing antibody activity compared with the viruses used to develop the vaccines. However, a booster dose significantly improved neutralizing antibodies, regardless of which vaccine was given in the initial immunization.
Katelyn Jetelina, PhD, an epidemiologist at the University of Texas Health Science Center at Houston, said that the found in this study confirm what researchers have already discovered in lab research -- vaccines offer significantly less protection against Omicron, and reinfection rates are expected to be high.
Jetelina noted that it was reassuring to see that "we can curb infection still with a booster, which is really quite phenomenal." But she also cautioned that cases are likely to increase.
"I think all this data is showing us that we're going to have a lot of infections with Omicron," Jetelina told 鶹ý. Although a high rate of infection does not necessarily equate to severe illness at the individual level, Jetelina said that she is concerned about population-level outcomes resulting from a flood of new cases.
"That's where I get a bit more nervous," she said. "Because a very transmissible variant will be sweeping across the United States, and even if the rate of severe disease is low, if you're talking about 330 million people, those numbers start adding up real quickly."
The U.K. Health Security Agency noted that these results should be interpreted with caution, due to the low number of Omicron cases included in their analysis. Additionally, more data are needed before scientists can determine how well vaccines will work against severe illness, hospitalization, and death from the Omicron strain.
"It will be a few weeks before effectiveness against severe disease with Omicron can be estimated," the agency stated. "However, based on this experience, this is likely to be substantially higher than the estimates against symptomatic disease."
The U.K. reported that cases from Omicron are doubling in the country every 2 to 3 days, and it reported the country's from the new strain.
Given the potential for an influx of Omicron cases here in the U.S., remaining vigilant in practicing public health interventions will be critical, said Jetelina.
"I think we still really need to curb transmission with masks, ventilation, and testing," she added. It is also extremely important that the "unvaccinated get vaccinated."