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One in 14 people admitted to hospital with coronavirus complications during the UK's second wave had received the first dose of a vaccine, a study has suggested.
The UK has three jabs in its immunisation arsenal against the pandemic – Pfizer-BioNTech, University of Oxford-AstraZeneca and Moderna, with the latter only recently being rolled out.
The three vaccines were approved after studies revealed they are up to 95% effective at warding off the coronavirus-related disease COVID-19.
With no jab offering complete protection, a Scientific Advisory Group for Emergencies (Sage) report suggests one in 14 people admitted to hospital with suspected or confirmed coronavirus between 8 December 2020 and 10 April 2021 had received "at least" the first jab in the vaccines' two-dose regimens.
Most of these patients are thought to have been infected shortly before or around the time of the vaccination, while others likely caught the virus before the jab had induced immunity, according to the scientists.
Overall, the team has called the results "broadly good news", adding "people shouldn't be surprised by some vaccine failure".
Officials and medics have repeatedly stressed vaccinated individuals should continue to wear face coverings, wash their hands regularly and maintain social distancing.
The preliminary results have not yet been published in a peer-reviewed journal.
A team of UK scientists analysed data submitted to the "coronavirus clinical characterisation consortium" ISARIC4C/CO-CIN.
The first Briton received a coronavirus jab on 8 December, which was followed by widespread vaccine roll out.
Between 8 December and 10 April, ISARIC4C/CO-CIN recorded 52,280 people were admitted to hospital with suspected or confirmed coronavirus.
Of these, 3,842 (7.3%) are known to have received at least the first vaccine.
In more than half (56%) of cases, information was available on the timing of ill-health relative to the patient being vaccinated, with symptoms generally emerging around nine days post-jab.
When infected with the coronavirus, symptoms tend to develop five days after being exposed to the pathogen.
The scientists therefore concluded "most vaccinated hospitalised patients were infected around the time of vaccination and the remainder after vaccination but before immunity had developed".
Just under three-quarters (74%) of the patients had information on the date of their vaccine relative to their hospital admission, generally 15 days.
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Overall, around two in five of the patients – described by the scientists as "an abundance" – became unwell within seven days of being vaccinated.
"This is too little time for the vaccination to work (immunity to develop) for most people and infection will have occurred before vaccination in many," wrote the team.
Elderly and vulnerable people – who were prioritised for a jab – may have "inadvertently been infected through the end-to-end process of vaccination"; the time between a vaccine being supplied by its manufacturer and administered to a patient.
Vaccinated people may have also changed their behaviour, "wrongly assuming they are immune".
Around one in five of the ISARIC4C/CO-CIN patients developed symptoms eight to 14 days post-vaccine.
"Admissions in this population remain unlikely due to vaccination failure, as immunity is not expected to have fully developed," wrote the scientists.
Professor Deborah Dunn-Walters – chair of the British Society for Immunology COVID-19 taskforce – agreed, adding: "We would not expect any vaccine to have much, if any, effect on disease hospitalisation rates before 14 days post-vaccination, as the immune system is still building its memory and protection will not yet be up to maximum power."
More than in 10 of the ISARIC4C/CO-CIN patients developed symptoms 15 to 21 days after their first vaccine dose.
"We have defined admissions in this population as vaccination failure only for the purpose of this analysis, as some immunity would be expected to have developed in some people," wrote the scientists.
Around a third became unwell more than 21 days post-vaccine, when "a greater degree of immunity would be expected".
It is also possible asymptomatic individuals may have been vaccinated, but later developed complications.
Of the 3,842 patients, more than nine in 10 (92%) swabbed positive for the coronavirus via the "gold standard" polymerase chain reaction (PCR) test. It is unclear if the remaining patients had a false-negative result or swabbed positive via the less accurate rapid lateral flow test.
Just under a quarter (23%) of those who swabbed positive via a routine PCR test were asymptomatic and admitted to hospital for non-COVID reasons.
Of these, more than one in 10 (15%) were admitted within seven days of being vaccinated, "indicating infection before immunity had opportunity to develop".
The ISARIC4C/CO-CIN scientists previously estimated one in 25 people hospitalised with suspected or confirmed coronavirus had received at least their first vaccine dose.
They pointed out, however, the study's participants are now being followed for longer, which creates the illusion more people are being admitted over 21 days after their jab.
Nevertheless, coronavirus cases are falling in the community, reducing exposure to the infection. This may lead to "under-representation of vaccine failure".
The scientists have also stressed not all coronavirus-related hospitalisations were included in their analysis.
The study participants were not analysed according to their age or ethnicity, with elderly and non-white people more at risk of COVID-19.
Overall, the scientists have stressed the number of vaccinated patients admitted to hospital is "very low".
Professor Dunn-Walters agreed, adding: "We can be confident the COVID-19 vaccination programme is highly effective in preventing serious disease and saving lives.
"I urge anyone who is offered a COVID vaccine to get it, however, let's not be complacent".
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