The effectiveness of the Pfizer-BioNTech vaccine against Covid-19 declines faster than that of the AstraZeneca jab, according to a new study published on Thursday.
"Two doses of Pfizer-BioNTech have greater initial effectiveness against new COVID-19 infections, but this declines faster compared with two doses of Oxford-AstraZeneca," researchers at Oxford University said.
The study, which has not been peer reviewed, is based on the results of a survey by Britain's Office for National Statistics that carried out PCR tests from December last year to this month on randomly selected households.
It found that "the dynamics of immunity following second doses differed significantly" between Pfizer and AstraZeneca, according to the university's Nuffield Department of Medicine.
Pfizer had "greater initial effectiveness" but saw "faster declines in protection against high viral burden and symptomatic infection", when looking at a period of several months after full vaccination, although rates remained low for both jabs.
"Results suggest that after four to five months effectiveness of these two vaccines would be similar," the scientists added, while stressing that long-term effects need to be studied.
The study's findings come as Israel is administering booster shots, after giving 58 percent of the population two shots of the Pfizer jab.
The United States is also set to offer booster vaccines to boost antibody levels following concerns over declining effectiveness of the Pfizer and Moderna vaccines.
The Oxford research also found that protection was higher among those who had already been infected with the virus.
The study examined two groups of more than 300,000 people over 18, first during the period dominated by the Alpha variant, which emerged in Kent, southeast England, and secondly from May 2021 onwards, when the Delta variant has been dominant.
It confirmed that vaccines are less effective against Delta, which was first seen in India.
The AstraZeneca vaccine is the most widely offered in the UK, while those under 40 are offered Pfizer or Moderna due to blood clotting concerns.