Vaccines against Covid-19 could still be effective despite a new study suggesting antibody levels fall "quite rapidly" after coronavirus infection, scientists leading the research have said.

The new study, from Imperial College London, involved more than 365,000 randomly selected adults who tested themselves at home using a finger prick test to check if they had antibodies against Covid-19.

Over this period, the proportion of people who tested positive for Covid-19 antibodies declined by 26.5%, suggesting antibodies reduce in the weeks or months after a person is infected.

Experts leading the Real-Time Assessment of Community Transmission (React-2) study said the findings suggested immunity was "waning quite rapidly", which could lead to an increased risk of reinfection.

However, Professor Wendy Barclay from Imperial, who worked on the study, said there was still reason to be optimistic about a vaccine being able to stimulate longer-lasting protection.

She told Times Radio today: "I think that we can still continue to be optimistic about vaccines because vaccines will work in a different way.

"What we're measuring at the moment is the way that our bodies' immune response reacts to the virus infecting us.

"But when we immunise with vaccines, particularly the new generation of vaccines that have been developed and put forward into trials for Sars-CoV-2, the virus that causes Covid, they work in quite different ways and they might make an immune response which is much more long lasting than natural infection.

"So we have to keep optimistic about that."

Her remarks come as it was revealed that there will not be sufficient doses of a coronavirus vaccine to cover the wider EU population before 2022.

If an effective antidote does become available, only a share of the 450 million people who live in the European Union will receive it before the end of 2021.

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Professor Paul Elliott, director of the React programme and also from Imperial, told BBC Radio 4's Today programme that a vaccine response "may behave differently to the response to natural infection".

Asked what the implications were for a vaccine and how long protection from a vaccine may last, he said: "I think that's an open question that needs to be kept under close scrutiny.

"It's possible that people might need booster vaccines.

"For some viruses there's lifelong immunity, for the coronaviruses that doesn't seem to be the case and we know that you know that that the immunity can fluctuate so, yes, this is something that needs to be looked at very carefully."

Professor Graham Cooke from Imperial, who also worked on the study, said: "The big picture is after the first wave, the great majority of the country didn't have evidence of protective immunity.

"The need for a vaccine is still very large, the data doesn't change that."

Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh, said the study suggested that home antibody testing kits were not useful for people trying to assess their own previous exposure.

She added: "These data should not be taken to infer that a vaccine would only induce short-term immunity.

"Vaccines contain immune stimulators (adjuvants) that induce durable immune responses and the administration of multiple doses of vaccine ensures that high concentrations of antibodies (that decline only slowly over time) are achieved in the majority of vaccine recipients."

Imperial's study, based on a survey of 365,000 randomly selected adults, was released as a pre-print paper, and has not yet been peer-reviewed.

World Health Organization spokesman Tarik Jasarevic said that uncertainty over how long immunity would last and the fact most people had never had antibodies against the coronavirus in the first place showed the need to break transmission chains.

"Acquiring this collective immunity just by letting virus run through the population is not really an option," he told a UN briefing in Geneva.