A single dose of the Pfizer/BioNTech coronavirus vaccine may block 75 per cent of symptomless infections, a study on NHS workers has suggested.
Asymptomatic screening of staff at Addenbrooke’s Hospital in Cambridge showed the number of people testing positive fell from 0.8 per cent to 0.2 per cent 12 days after their injection.
The ‘very impressive’ findings are another sign that the UK’s gamble to delay the second dose in a bid to get wider coverage quicker has paid off.
Curbing symptomless infections is crucial to stopping outbreaks from growing unknowingly because people who do not feel ill are less likely to self-isolate.
Numerous studies had indicated that a single injection of the Pfizer jab prevented severe illness and death but to what degree it blocked asymptomatic spread was unclear. Research has also suggested Oxford University’s jab stops more than 60 per cent of asymptomatic cases.
The study, led by Cambridge University, analysed thousands of weekly Covid tests at Cambridge University Hospitals NHS Foundation Trust in January.
It found that 26 out of 3,252 swabs (0.8 per cent) from unvaccinated workers yielded a positive result, compared to four out of 1,989 (0.2 per cent) in staff 12 days or more after their vaccine.
The study, which is yet to be peer-reviewed, also found asymptomatic infection was halved in those vaccinated for less than 12 days, suggesting immunity kicks in rapidly in many cases.
Independent experts reacting to the pre-print hailed the findings, saying they signalled Britain was headed for a ‘much brighter future’.
A single dose of the Pfizer/BioNTech coronavirus vaccine may stop 75 per cent of people spreading the disease, a study on NHS workers has suggested
It found that 26 out of 3,252 swabs (0.8 per cent) from unvaccinated workers yielded a positive result, compared to four out of 1,989 (0.2 per cent) in staff 12 days or more after their vaccine
Professor Jonathan Ball, a molecular virologist at the University of Nottingham, said: ‘To see such a reduction in infection rates after a single dose of the Pfizer vaccine is very impressive and shows that vaccination truly does offer a way out of the current restrictions and a much brighter future.
‘It will be important to understand whether the reduced risk of infection played out across all the exposure risk groups included in the study, but nonetheless, this is still excellent news.’
Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, added: ‘In working age adults, the Pfizer-BioNTech vaccine has good efficacy for preventing asymptomatic infection by the Covid-19 coronavirus.
‘This is really important if we are to decrease the amount of spread from people who are unaware that they are infected.
‘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.’
He added the study ‘relies on studying NHS staff and there appears to have been no assessment made of their pre-existing immune status, which might have been higher than in the general population given healthcare workers’ likelihood of exposure at work.
‘Pre-exposure to the coronavirus before receiving a dose seems to cause a more potent immune response to the vaccine.
‘It’s also important to remember that these findings cannot automatically be extended to every other vaccines.’
During a two-week period between January 18 and 31, Cambridge researchers screened similar numbers of vaccinated and unvaccinated staff using around 4,400 PCR tests per week.
A positive PCR swab would signal that even someone who is vaccinated is carrying fragments of the virus in their nose or throat which they could pass on.
Reducing Covid’s spread is critical to achieve ‘herd immunity’, when so many people are immune that a disease peters out.
The results were then separated out to identify unvaccinated staff, and staff who had been vaccinated more than 12 days prior to testing, when immunity kicks in.
When the team included symptomatic healthcare workers, their analyses showed similar reductions.
Some 56 out of 3,282 (1.7 per cent) of unvaccinated healthcare workers tested positive, compared to eight out of 1,997 (04 per cent)at 12 or more days post-vaccination, a fourfold reduction.
The researchers have released their data ahead of peer review because of the urgent need to share information relating to the pandemic.
Dr Mike Weekes, an infectious disease specialist at the trust, who led the study, said: ‘This is great news – the Pfizer vaccine not only provides protection against becoming ill from SARS-CoV-2 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 that 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.’
Dr Nick Jones, first author on the study and an infectious diseases expert at Cambridge, added: ‘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.’
The UK drew criticism in January when it pushed back giving second doses of both vaccines from three weeks to three months because the jabs were never trialled using that dosing regimen.
The policy recommendation was made by the Joint Committee on Vaccination and Immunisation (JCVI) as the country grappled with the highly virulent Kent variant and limited vaccine supplies. Its goal was to get wider coverage more quickly.