A “very low” number of vaccinated patients are being admitted to hospital, scientists have said, in what is seen as confirmation that the Covid-19 jabs are working as expected within the community.
The clinical trials used to assess the safety and effectiveness of the vaccines showed that they are capable of offering high levels of protection against infection, hospitalisation and death.
However, a small minority of people – particularly those who are frail, elderly and immunologically compromised – will not acquire protection against Covid-19 through vaccination, the trials demonstrated.
This is being represented in real-world data accumulated and assessed by scientists. For now, it is specific to patients who had only received one vaccine dose.
Analysis conducted by the government’s Scientific Advisory Group for Emergencies (Sage) has found that out of 52,000 patients who were admitted to hospital between 8 December and 10 April, 3,598 had received the first dose of a Covid-19 vaccine.
For most of these patients, the time between vaccination and hospital admission was up to 15 days, where infection occurred before immunity had developed, the UK study showed.
Three weeks is the point at which experts believe maximum immunity from a single dose becomes apparent.
The researchers said that there is a possibility that vaccinated elderly and vulnerable people may have been been exposed to the coronavirus after wrongly assuming they were immune.
Calum Semple, professor in child health and outbreak medicine at the University of Liverpool, who co-led the study, said: “Many people are acquiring Covid within two weeks of being vaccinated, which does indicate that people are letting their guard down.
“There is evidence here that people are, unfortunately, assuming that they are protected very quickly after vaccination and that is not the case.”
Of those vaccinated, 526 people with a positive Covid-19 test were admitted to hospital more than 21 days after receiving their first dose, and 113 died.
The experts said these cases represent vaccine failure, which occurs when a person contracts a disease in spite of being vaccinated against it.
Dr Annemarie Docherty, an honorary consultant in critical care at the University of Edinburgh, and study co-author, said: “Our take-home messages are that this is some real world data, which does show some vaccine failure, but this is not unexpected, and reflects what the [clinical] trials have shown.”
However, she added that their data may be under-representing vaccine failure, as the risk of Covid-19 exposure has reduced since early January when infections were at a peak.
Dr Docherty said: “The one caveat is that [while] lots of people have been vaccinated recently, their risk of exposure to Covid is much lower than it was in January and February.
“So whilst we think this is good news, potentially we are under-representing vaccine failure, because people aren’t exposed to the virus.”
She said their study only recruited patients who were admitted to hospital and may not reflect vaccine effectiveness within the wider community.
Prof Semple said the group’s analysis was designed to give “a very early signal on whether vaccines were working in the real world”.
He said: “The absolute numbers of people being admitted to hospital after 21 days of the first dose is tiny.
“It is very small, but it does occur – it is mostly occurring in the group most at risk of severe disease, which is the elderly.
“This is good real world evidence of it working, but there are some few failures.
“And when these failures do occur, sadly, people die, but that is because they are elderly and frail.”