New data from the UK indicates that the effectiveness of Pfizer vaccine booster shots in preventing symptomatic COVID-19 as a result of the Omicron variant drops considerably within 10 weeks of receiving the inoculation.
But it also showed that protection afforded by Moderna boosters remained relatively strong for a longer period.
The UK Health Security Agency (UKHSA) reviewed 68,489 Omicron cases in the country. It assessed that Pfizer and Moderna vaccines were around 70 percent effective at preventing symptomatic disease 2-4 weeks after getting a third shot (down from some 90% effective against the previously dominant Delta variant)
With Pfizer boosters, this dropped to 45% by 10 weeks, but Moderna booster effectiveness stayed at around 70% for at least 9 weeks.
In Israel, the vast majority of people were given Pfizer boosters.
Reacting to the findings, Jenny Harries, head of the UKHSA, said boosters were still very important and would help prevent severe cases and deaths.
“Despite the headlines people need to really understand that a booster dose is absolutely vital,” Harries told the BBC. “We anticipate it will have a very significant positive impact on preventing serious illness and death.”
Notably, most symptomatic Omicron cases are mild. Recent data from the UKHSA has indicated the risk of hospitalization from the strain is 50%-70% lower than Delta.
The analysis follows two studies, from Imperial College London and Scottish researchers, that found patients with Omicron were between 20-68% less likely to require hospital treatment than those with Delta.
Even if the early studies are borne out, the new variant could still overwhelm health systems because of the sheer number of infections.
Countries around the world are looking closely at Britain, where Omicron is now dominant and where COVID-19 cases have surged by more than 50% in a week.
Britain reported 119,789 lab-confirmed COVID-19 cases on Thursday, the highest yet during the pandemic and the second day the number has topped 100,000.