One Pfizer coronavirus vaccine dose cuts asymptomatic infections, study suggests

·5-min read

Watch: One dose Pfizer coronavirus vaccine wards off asymptomatic transmission

A single dose of the Pfizer-BioNTech coronavirus vaccine reduces the risk of asymptomatic infections, helping to stem transmission, research suggests.

The UK has approved three coronavirus jabs – Pfizer-BioNTech, AstraZeneca-University of Oxford and Moderna – after studies demonstrated they significantly ward off severe disease.

With not everyone able to be immunised, or producing a strong immune response as a result of their vaccination, questions lingered as to whether the jabs prevent the infection itself.

Asymptomatic infections are particularly important amid the pandemic, given these individuals do not know to isolate and may unwittingly transmit the coronavirus when they speak or even breathe.

To learn more, medics from the Cambridge University Hospitals NHS Foundation Trust (CUH) swabbed the site's vaccinated and unvaccinated healthcare workers over two weeks.

Read more: First coronavirus vaccine dose cuts hospital admission risk by up to 94%

Results suggest the risk of an asymptomatic infection decreased by four times more than 12 days after a worker received their first dose of the Pfizer-BioNTech jab.

The vaccine was approved based on a three-week interval between the two doses, which has been extended to up to 12 weeks to maximise the number of first jabs being administered.

Vaccination of senior person in hospital
The coronavirus vaccines were approved after they were found to protect against severe disease, with their effect on coronavirus transmission being less clear. (Stock, Getty Images)

The results have been released preliminarily and are yet to appear in a peer-reviewed journal.

"This is great news," said lead author Dr Mike Weekes.

"The Pfizer vaccine not only provides protection against becoming ill from SARS-CoV-2 [the coronavirus] but also helps prevent infection, reducing the potential for the virus to be passed on to others.

"This will be welcome news as we begin to plot a roadmap out of the lockdown, but we have to remember the vaccine doesn't give complete protection for everyone.

"We still need social distancing, masks, hand hygiene and regular testing until the pandemic is under much better control."

Read more: 12 criteria to meet ahead of coronavirus vaccine passports

Experts have been unanimously optimistic the three approved vaccines will cut transmission to some extent, despite this not being backed up by hard data when they were officially signed off.

Data has since suggested the AstraZeneca-Oxford vaccine may also lead to a "substantial" fall in the coronavirus' spread.

If the jabs only stopped people from becoming seriously ill with the infection, then theoretically everyone would need to be vaccinated.

Evidence they stem transmission means each person who is immunised indirectly protects others.

Unable to marshal the right cells and molecules to fight off the invader, the bodies of the infected instead launch an entire arsenal of weapons — a misguided barrage that can wreak havoc on healthy tissues, experts said. (Getty Images)
The coronavirus may spread when people with an asymptomatic infection talk or breathe. (Stock, Getty Images)

CUH officials began vaccinating the trust's healthcare workers on 8 December 2020, with mass immunisation from 8 January 2021.

Between 18 and 31 January, the team screened the trust's vaccinated and unvaccinated staff, with around 4,400 swabs being performed every seven days of the two-week period.

The results reveal 26 (0.8%) of the unvaccinated staff tested positive for the coronavirus, without developing symptoms.

This is compared to 13 (0.37%) of the workers who had their jabs less than 12 days earlier, around the time the infection-fighting antibodies kick in.

Just 0.2% of the employees tested positive for the coronavirus 12 or more days post-vaccination. No jab is completely effective, with the immune response varying between individuals.

Overall, the results suggest a four-fold reduction in the risk of asymptomatic coronavirus infection among the healthcare workers who had been vaccinated more than 12 days before.

Asymptomatic transmission also halved in those who had been immunised for less than 12 days.

"We therefore provide real-world evidence for a high level of protection against asymptomatic SARS-CoV-2 infection after a single dose of [the Pfizer-BioNTech] vaccine, at a time of predominant transmission of the UK COVID-19 [the disease caused by the coronavirus] variant of concern [the so-called Kent variant] and amongst a population with a relatively low frequency of prior infection," wrote the scientists.

Less than one in 10 (7.2%) of the study's participants had antibodies against the coronavirus, which indicate a prior infection.

Read more: I felt guilty for having the coronavirus vaccine

"Our findings show a dramatic reduction in the rate of positive screening tests among asymptomatic healthcare workers after a single dose of the Pfizer-BioNTech vaccine," said study author Dr Nick Jones.

"This is fantastic news for both hospital staff and patients, who can be reassured the current mass vaccination strategy is protecting against asymptomatic carriage of the virus in addition to symptomatic disease, thereby making hospitals even safer places to be."

Research suggests people have put off going to hospital over fears of catching the coronavirus, with some even ignoring cancer symptoms.

"Throughout the pandemic so far, we have taken a systematic approach to keeping our staff safe and well," said Giles Wright, programme director for the CUH vaccination hub.

"The huge efforts of all those involved in the testing, tracing and vaccination programmes at CUH are making the plan a reality.

"We are very encouraged by the findings of our research. It gives further hope for the near future."

When the medics focused on symptomatic infections, the results were similarly encouraging.

Of these unvaccinated workers, 1.7% swabbed positive for the coronavirus, compared to 0.4% of those who were immunised 12 or more days earlier – a 4.3-fold reduction.

Speaking of the results, Dr Simon Clarke from the University of Reading said: "While this is very encouraging, no data were provided to show how long the effect will last for and continued surveillance is required in case this protection dwindles."

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