LONDON (AP) — Warning that the first COVID-19 vaccines may be imperfect, the head of the U.K effort to develop a vaccine called Wednesday for immediate international cooperation to prevent the “largest global recession in history.’’

U.K. Vaccine Taskforce chair Kate Bingham also warned against over-optimism, saying there is no guarantee a successful vaccine against the novel coronavirus will ever be developed.

“The first generation of vaccines is likely to be imperfect, and we should be prepared that they might not prevent infection, but rather reduce symptoms, and, even then, might not work for everyone or for long,’’ Bingham wrote in an essay published in The Lancet medical journal.

No vaccine has ever been developed against any coronavirus, and numerous attempts to design vaccines against SARS and MERS, two which are related to the virus that causes COVID-19, have failed. Scientists also warn that immunity against coronaviruses appears to fade over time and that achieving any vaccine-induced immunity to protect against infection or severe disease could be challenging.

Bingham’s comments come as government leaders in Britain and other countries are basing their COVID-19 strategies on expectations of a vaccine becoming available as soon as early next year.

Prime Minister Boris Johnson, for example, has implemented a three-tiered regional strategy aimed at slowing the spread of the disease until there is a vaccine while also minimizing the economic damage from another widespread lockdown.

Meanwhile, coronavirus cases, hospitalizations and deaths in the U.K. and elsewhere in Europe are rising amid a second wave of infections.

Britain created the Vaccine Taskforce earlier this year to speed the development of a vaccine. Two candidates are in late stage trials, with results expected around the end of the year or early 2021. Dozens of other potential vaccines are in earlier stages of development.

But Taskforce chair Bingham said the candidates that have progressed the most in testing are based on new approaches with little history of being used in vaccines. Vaccine candidates based on more frequently used methodologies aren’t likely to be ready until late 2021, she added.

In addition, it will be a challenge to rapidly produce enough vaccines to combat the virus. The global manufacturing capacity is “vastly inadequate for the billions of doses that are needed,” she wrote in The Lancet.

“China, Europe, the USA and the U.K. need to work together,’’ Bingham wrote. “If we establish international collaboration right now, then we will be better prepared to control future pandemics without causing the largest global recession in history and the biggest threat to lives in living memory.’’

The World Health Organization says it hopes first-generation coronavirus vaccines will have an efficacy rate of at least 70%, but it has acknowledged that 50% might be more realistic. The European Medicines Agency has not specified any minimum efficacy threshold for COVID-19 vaccines and said they would be evaluated on a case-by-case basis.

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Maria Cheng contributed to this story.

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