Vaccines are effective enough at preventing severe cases of Covid-19 that there is no current need for the general population to be given third doses, according to a report in The Lancet published Monday.

Some countries have started offering extra doses over fears about the much more contagious Delta variant, causing the World Health Organization to call for a moratorium on third jabs amid concerns about vaccine supplies to poorer nations, where millions have yet to receive their first jab.

The new report by scientists, including from the WHO, concluded that even with the threat of Delta, "booster doses for the general population are not appropriate at this stage in the pandemic".

The authors, who reviewed observational studies and clinical trials, found that vaccines remain highly effective against severe symptoms of Covid-19, across all the main virus variants including Delta, although they had lower success in preventing asymptomatic cases of the disease.

"Taken as a whole, the currently available studies do not provide credible evidence of substantially declining protection against severe disease, which is the primary goal of vaccination," said lead author Ana-Maria Henao-Restrepo, of the WHO.

She said vaccine doses should be prioritised to people around the world still waiting for a jab.

"If vaccines are deployed where they would do the most good, they could hasten the end of the pandemic by inhibiting further evolution of variants," she added.

Vaccine divides

Countries like France have started distributing third jabs to the elderly and people with compromised immune systems, while Israel has gone further, offering children 12 and older a third dose five months after receiving a second jab.

WHO chief Tedros Adhanom Ghebreyesus has called on countries to avoid giving out extra Covid jabs until the end of the year as the UN health agency urges all nations to vaccinate at least 10 percent of their populations by the end of this month, and at least 40 percent by the end of this year.

The Lancet study concluded that the current variants had not developed sufficiently to escape the immune response provided by vaccines currently in use.

The authors argue that if new virus mutations do emerge that are able to evade this response, it would be better to deliver specially modified vaccine boosters aimed at the newer variants, rather than a third dose of an existing vaccine.

Commenting on the study Azra Ghani, Chair in Infectious Disease Epidemiology, at Imperial College London, described it as a "very thorough review" of current research.

But she said that while the reduction in efficacy of vaccines against variants like Delta might be small, when considered across a population it could still lead to "a substantial increase" in people needing hospitalisation.

"Even in the most developed countries, these small differences can put a severe strain on the health system," she said in a statement to the Science Media Centre, adding there was no "one size fits all" approach to booster vaccines.