A single dose of the Pfizer vaccine can reduce asymptomatic infections by 75%, according to research that suggests the jab could substantially curtail transmission of the disease.

Doctors in Cambridge recorded the sharp fall in infections after 12 days of the first shot in an analysis of Covid tests performed on healthcare workers in the last two weeks of January.

Pfizer/BioNTech

Country US/Germany

Efficacy 95% a week after the second shot. Pfizer says it is only 52% after the first dose but the UK’s Joint Committee on Vaccination and Immunisation (JCVI) says this may rise to 90% after 21 days.

The UK has ordered 40m doses.

Doses Clinical trials involved two doses 21 days apart. The UK is stretching this to 12 weeks.

Oxford/AstraZeneca

Country UK

Efficacy 70.4% 14 days after receiving the second dose. May have up to 90% efficacy when given as a half dose followed by a full dose. No severe disease or hospitalisations in anyone who received the vaccine. There have been concerns it is less effective against the South African variant of the coronavirus.

The UK has ordered 100m doses.

Doses Two, four to 12 weeks apart

Moderna

Country US

Efficacy Phase 3 trial results suggest 94.1%.

The UK has ordered 17m doses, to be delivered in March or April

Doses Two, 28 days apart

Novavax

Country US

Efficacy Phase 3 trials suggest 89.3%.

60m doses ordered by the UK, with distribution expected principally in the second half of the year

Doses Two

Janssen (part of Johnson & Johnson)

Country US

Efficacy 72% in preventing mild to moderate cases in US trials but 66% efficacy observed in international trials. 85% efficacy against severe illness, and 100% protection against hospitalisation and death.

30m doses ordered by the UK

Doses: One, making it unique among Covid vaccines with phase 3 results so far


Photograph: Stéphane Mahé/X02520

The findings build on similarly positive results from Public Health England’s Siren study, which found infections in healthcare workers, some with symptoms and some without, fell by 70% after one dose of the Pfizer vaccine, and by 85% after two. Earlier work found evidence that a single shot of the AstraZeneca vaccine reduced transmission by about 67%.

While clinical trials demonstrated that both vaccines prevented people falling sick with Covid, it was unclear to what extent the shots would also protect people from becoming infected with the virus. Those who have the virus but no symptoms are a major risk for spreading the disease.

“Two weeks after a single dose of the Pfizer vaccine, we saw a 75% reduction in asymptomatic infections with coronavirus in healthcare workers. We saw a similar reduction in symptomatic infections. I’d anticipate a comparable effect in the community,” said Mike Weekes, an infectious disease specialist at Cambridge University Hospitals NHS Foundation Trust (CUH) and Cambridge University, who led the study.

“This will mean a substantial reduction in transmission of the virus as more and more people are vaccinated, which is really great news.”


The hospital began vaccinating staff on 8 December. Between 18 and 31 January, the researchers tested similar numbers of vaccinated and unvaccinated staff for Covid using PCR lab tests. The team then compared infection rates among unvaccinated staff and those who had received the jab more than 12 days before testing – the time it takes for the body to mount a good immune response.

The study, which has yet to be peer reviewed, found that 26 out of 3,252, or 0·8%, of tests from unvaccinated healthcare workers were positive, compared with four out of 1,989, or 0.2%, of tests from healthcare workers who had a jab at least 12 days before testing. A further 13 out of 3,535, or 0.37%, of tests were positive from healthcare workers less than 12 days after having the shot.

Despite the encouraging results, the study shows that a single shot of the vaccine does not completely prevent infections, leading the authors to stress the importance of continuing to follow Covid guidelines such as social distancing, mask wearing and good hand hygiene.

“This reinforces the government’s roadmap to easing lockdown,” Weekes said. “Although vaccination had a substantial effect, there were nevertheless still some infections in vaccinated people. This means that at present, while there are still a large number of daily cases, social distancing, mask wearing, hand washing and regular testing are as important as ever.

“If you have been offered the opportunity to have any of the vaccines for Covid-19, you should do so as soon as possible. This data provides further evidence that they are highly effective, and will protect you, your loved ones, your friends and your colleagues.”

Nick Jones, first author on the study and an infectious diseases and microbiology registrar at CUH, said: “This is fantastic news for both hospital staff and patients, who can be reassured that 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.”

Test results from healthcare workers who developed symptoms of Covid showed similar reductions after a single shot of the Pfizer vaccine. While 56 out of 3,282, or 1·71%, or the unvaccinated healthcare workers tested positive, only eight out of 1,997, or 0·4%, of healthcare workers at 12 or more days post-vaccination tested positive, a more than fourfold reduction in symptomatic infections.



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