LONDON • Covid-19 vaccines work so well that most people do not yet need a booster, an all-star panel of scientists from around the world said in a review that is likely to fuel debate over whether to use them.
Governments would be better off focusing on the unvaccinated and to wait for more data on which boosters is most effective and at what doses, the authors, who included two prominent US Food and Drug Administration (FDA) experts, argued in the medical journal The Lancet.
They based their assessment on a wide range of real-world observational studies as well as data from clinical trials.
"None of the studies has provided credible evidence of substantially declining protection against severe disease," they wrote.
There could be more side-effect risks if boosters are introduced too soon or too broadly, they said.
Most countries with ample vaccine supplies are debating whether to allocate doses for booster shots to prop up immunity and potentially help stop the spread of the more infectious Delta variant.
The United States plans to roll out booster shots starting on Monday, though the proposal still needs approval from the FDA and the Centres for Disease Control and Prevention (CDC).
Scientists are by no means unanimous on the topic of boosters.
Even a small reduction in efficacy against the spread of Covid-19 can strain a healthcare system, and "there is therefore no 'one size fits all' approach", said Professor Azra Ghani, chair in infectious disease epidemiology at Imperial College London, who was not involved with the review.
The analysis is a blow to US President Joe Biden, who announced his booster programme last month after an extraordinary joint statement from Dr Anthony Fauci, chief medical adviser to the White House, other advisers, including CDC director Rochelle Walensky, and FDA acting commissioner Janet Woodcock.
The European Medicines Agency is also reviewing booster data from Pfizer and BioNTech, and from Moderna.
Some experts have questioned the need for extra shots, while the World Health Organisation (WHO) has called for a moratorium on them until more people in lower-income countries can get protection.
Among the scientists behind the Lancet article were Dr Marion Gruber, who leads the FDA's vaccines office, and her deputy, Dr Philip Krause. Both have said they would step down later this year.
The WHO's chief scientist Soumya Swaminathan, Dr Ana-Maria Henao-Restrepo and top emergencies expert Mike Ryan also worked on the review.
The WHO said it would make better public health sense to focus on immunising those who have not yet had any shots - whether because of anti-vaccine sentiment in countries with ample reserves, or because they live in places with little access to shots.
"Even if boosting were eventually shown to decrease the medium-term risk of serious disease, current vaccine supplies could save more lives if used in previously unvaccinated populations," the authors wrote.
Across the observational studies done so far, vaccination has shown an average of 95 per cent effectiveness against severe disease, including against more infectious variants such as Delta, and more than 80 per cent effective at preventing any infection, the review found.
Even in countries with high vaccination rates, it is unvaccinated people who are driving transmission of the virus - and who are at highest risk of becoming very ill, the study found.