James Shaw, 82, receives Oxford University / AstraZeneca COVID-19 vaccine from advanced nurse Justine Williams on January 4, 2021 at Lochee Health Center in Dundee, Scotland, UK.

Andy Buchanan | Reuters

LONDON – A single dose of the Covid-19 vaccine from Oxford-AstraZeneca or Pfizer-BioNTech drastically reduces the risk of infection in adults of all ages, British researchers have found.

Two studies published on Friday analyzed more than 1.6 million nasal and throat swabs from 373,402 people between December and April. The data was collected as part of the ongoing Covid-19 infection survey carried out by Oxford University, the UK Office for National Statistics and the UK Department of Health and Welfare.

The researchers found that 21 days after a single dose of the Oxford-AstraZeneca or Pfizer-BioNTech vaccine, new Covid infections – both symptomatic and asymptomatic – had decreased by 65%.

Symptomatic infections decreased by 74% three weeks after a single dose of either vaccine, while asymptomatic cases decreased by 57%, the data showed.

A second dose of vaccine reduced the overall infection rate by 70%, reducing symptomatic Covid infections by 90% and asymptomatic cases of the virus by 49%.

The researchers compared these effects to the natural immunity obtained from infection with the virus.

However, they warned that the fact that vaccinated people could still be infected – even if those infections were mostly asymptomatic – meant “transmission possible”.

The study found that vaccines had a similar effect in reducing infection rates in adults of all ages. Their ability to reduce infection was also similar, regardless of whether the participants had long-term health conditions or not.

What about antibody resistance?

The scientists also looked at the effects of Covid vaccinations on participants’ antibody levels.

They found that older adults – especially those over 60 – who had never been infected with Covid had a lower immune response to a single dose of vaccine than those who had previously been infected with the virus.

Antibody responses to two doses of the Pfizer BioNTech vaccine were high in all age groups. The data showed that older adults were able to achieve antibody levels similar to those who received a vaccine dose after a previous Covid infection.

Too few people in the UK had received two doses of the Oxford AstraZeneca vaccine for researchers to assess the effects on antibody response. However, it was found that immune responses to a first dose differed between the Oxford AstraZeneca vaccine and the Pfizer BioNTech vaccine.

Antibody levels rose more slowly after a single dose of the Oxford-AstraZeneca vaccine than after the Pfizer-BioNTech alternative. However, after a dose of the latter, antibody levels fell more rapidly, especially in older adults, so patients achieved antibody levels similar to those seen after an initial dose of the Oxford-AstraZeneca vaccine.

Although immune responses differed between age groups, the scientists emphasized that there was no group that did not respond to either vaccine. However, a small number of people – less than 5% – had poor immune responses to both vaccines.

Important to get the second dose

The Oxford-AstraZeneca vaccine has been approved for use in the UK, India and several other countries, but has been temporarily suspended in some markets amid concerns that it could be linked to rare blood clots. Global health officials have stated that the benefits of giving the vaccine continue to outweigh the risks.

The WHO recommends an interval of eight to 12 weeks between the first and second dose of the Oxford AstraZeneca vaccine.

The Pfizer BioNTech vaccine is also given in several countries, including the United States. The U.S. Centers for Disease Control and Prevention recommends receiving a second dose of the vaccine three weeks after the first.

In February, the UK started a study to see if mixing doses of the Oxford-AstraZeneca and Pfizer-BioNTech vaccines could be effective.

Sarah Walker, professor of medical statistics and epidemiology at Oxford University and chief investigator and academic director of the Covid-19 infection survey, said Friday that scientists are still not sure how strong and how long an antibody response is. was needed for long-term protection against Covid.

David Eyre, associate professor at Oxford University’s Big Data Institute, added that the results released on Friday highlighted the importance of a second dose of vaccine for increased protection.