Scientists behind the research said on Friday that there was evidence of rare instances of inoculated people still developing severe Covid-19 symptoms.

Most of the small number of people in this situation were likely infected soon after they received the first dose of the vaccine, before the body had time to fully develop immunizing antibodies against the virus, the researchers said.

The study, published in a preprint journal on Friday without being peer-reviewed, used data from 52,000 hospitalised Covid-19 patients.

The cohort included 526 patients who had received a first dose of a vaccine made by AstraZeneca or Pfizer in the previous three weeks. Out of this group 113 people died, most of whom were elderly or clinically vulnerable.

The study’s co-author, Professor Calum Semple of the University of Liverpool, presented the findings on Friday, saying: “We’re not saying the vaccine doesn’t work. In fact, this is good real-world evidence of it working. But it also shows that the vaccine isn’t perfect.”

He said the cases of people being infected so soon after getting their jabs indicate that “people are letting their guard down because they’ve been vaccinated.”

On Friday, another UK study also suggested that vaccination alone does not determine a person’s resistance to Covid-19, and that although jabs do offer protection, they may not give full coverage against infection.

The research, published in the journal Science, found that people previously infected with Covid-19 who were injected with a single dose of the Pfizer vaccine showed a stronger immune response against virus variants.

Those who previously had mild or asymptomatic infection had “significantly enhanced protection” against the UK and South Africa variants, according to researchers from Imperial College London, Queen Mary University of London, and University College London.

In people without prior infection, the immune response was “less strong” after a first dose, potentially leaving them more susceptible to variants.

One of the study’s co-authors, Professor Rosemary Boyton of Imperial College, said the data showed that “natural infection alone” may not provide sufficient immunity against variants.

“People who have had their first dose of vaccine, and who have not previously been infected with SARS-CoV-2, are not fully protected against the circulating variants of concern,” she said in a statement.