In a pinch, a single shot certainly helps.

A first look at the potential effect of stretching limited COVID-19 vaccine supplies has found that just one dose of the Pfizer-BioNTech vaccine was 60% to 70% effective at preventing symptomatic disease in people age 70 and older.

That protection started 10 to 13 days after receiving the shot and lasted for more than six weeks, according to a preliminary report released Tuesday by Public Health England.

The study allowed the health service to gauge the effects of the country’s unorthodox vaccination strategy. Faced with limited doses, surging infection rates and a new strain that was spreading about 56% more readily than its predecessors, British officials opted to immunize larger numbers of people with a first dose rather than maintain strict adherence to the set schedule of giving first and second doses three weeks apart.

Researchers from Public Health England, which advises the United Kingdom’s National Health Service, tracked roughly 157,000 Britons  70 and over who were tested for COVID-19 between Dec. 9 and February 19. Their vaccination status and health outcomes were compared at different intervals to determine how protective vaccine was and when those effects became apparent.

The study authors found that a month after Brits  80 and over got a first dose of the Pfizer-BioNTech vaccine, they were 43% less likely to be admitted to the hospital with COVID-19 than their unvaccinated peers. They were also 51% less likely to die of of the disease.

In cases where people over 70 received a second dose of the vaccine, the risk of developing any type of COVID-19 symptoms was reduced by 85% to 90% – a level of protection in line with that seen in clinical trials. That benefit was apparent even if the time between the first dose and second was more than the three weeks originally planned.

There was more good news. The study found that these effects were undiminished by the widespread presence of the United Kingdom’s more transmissible coronavirus strain, dubbed B.1.1.7 by scientists.

Finally, the encouraging results were all seen in Britons over the age of 80. This group – which got first dibs on vaccine in the U.K. and was there therefore first to be studied – has been particularly vulnerable over the course of the pandemic. Their robust response to inoculation was also heartening because people of this age do not always mount an effective response to vaccines, experts noted.

The new research gives the British government greater confidence that there were few risks, and potentially many benefits, to allowing more than three weeks to elapse between the administration of first and second doses of the Pfizer vaccine.

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