The rush to immunise populations against Covid-19 could lead to the rollout of a vaccine that is not very effective and risk worsening the pandemic, leading scientists have said.

Experts, members of the WHO’s Solidarity Vaccines Trial Expert Group, which is made up of leading scientists around the world who are advising on the establishment of the WHO’s trial to compare different vaccine candidates, in a paper published on The Lancet say strong evidence of efficacy needed to avoid approval of inferior vaccines.

The group said that a poor vaccine would be worse than no vaccine, not least because people who had it would assume they were no longer at risk and stop social distancing.

“Deployment of a weakly effective vaccine could actually worsen the Covid-19 pandemic if authorities wrongly assume it causes a substantial reduction in risk, or if vaccinated individuals wrongly believe they are immune, hence reducing implementation of, or compliance with, other Covid-19 control measures,” they said.

They urged all regulators to stick to the WHO’s guidance, which says that no vaccine that is less than 30% effective should be approved. It recommends at least 50% effectiveness, but allowing for 95% accuracy that could mean 30% in practice.

The Food and Drug Administration, the US regulator, has said it will abide by the 30% guidance, but some observers think it may come under political pressure to license a vaccine that falls below that threshold.

The Lancet piece says trials such as Solidarity, which compare various vaccines, are a better way to proceed.

“In comparison with individual trials for each of the many different vaccines, a global multi-vaccine trial with a shared control group could provide more rapid and reliable results,” they write.

“High enrolment rates facilitated by flexible trial design and hundreds of study sites in high-incidence locations could yield results on short-term efficacy for each vaccine within just a few months of including that vaccine.”

Good results on safety and effectiveness do not guarantee there will be no long-term problems, they add. The vaccine’s protection may wane, for instance.

“The trial costs will be a fraction of the societal costs of Covid-19, and this global collaboration could rebut detrimental vaccine nihilism and vaccine nationalism,” they write.