Senior citizens who received a second booster of the Pfizer–BioNTech COVID-19 vaccination had a 78% lower mortality rate from the disease than those who got one only, a study from Israel showed on Sunday.

The country's largest healthcare provider, Clalit Health Services, said the 40-day study included more than half a million people aged 60 to 100.

Some 58% of participants had received a second booster - or two shots in addition to the basic two-shot regimen. The remainder had received only one booster. Researchers recorded 92 deaths among the first group and 232 deaths among the second, smaller group.

"The main conclusion is that the second booster is lifesaving," said Ronen Arbel, Health Outcomes Researcher at Clalit and Sapir College.

The report was issued as a preprint and has not been peer-reviewed. The research excluded people who received rival Moderna's vaccine and those who had taken oral anti-COVID therapy.

Israeli health officials have put out a number of studies on vaccine efficacy throughout the pandemic that have impacted policymaking in other countries.