Washington (AFP)

Can the second dose of the Covid-19 vaccine be taken later than initially recommended?

Could we administer half doses?

And is using a different vaccine between the first and second dose as effective?

Governments around the world are now faced with these questions in the face of a raging epidemic, and the appearance of variants that are a priori more contagious.

The British authorities set the tone, announcing that they would delay the administration of the second dose of the vaccines, until three months after the first, well beyond the recommended three to four weeks.

The goal: to ensure that more people receive an injection more quickly, even if the level of protection is lower than that acquired after receiving both doses.

The World Health Organization supported the British position on Tuesday, estimating that the second injection could be delayed by a few weeks "in exceptional circumstances of epidemiological contexts and supply constraints".

And if the vaccine used for a first injection is no longer available, the British authorities have also authorized the administration of a different vaccine for the second.

The United States, on the contrary, has taken a much more cautious line.

On Monday evening, Drugs Agency (FDA) chief Stephen Hahn said these were "reasonable issues to consider and assess," but that the changes were "premature" and were "not firmly anchored in available evidence ".

- "Slight changes" -

The debate highlighted divisions among experts, with prominent scientists taking opposing positions.

Ideally, public policies should follow what has been tested in clinical trials.

But with an epidemic threatening to spiral out of control, for some the situation is far from ideal.

"We didn't pick three weeks for (the) Pfizer vaccine and four for Moderna because we know it's perfect," said Howard Forman, a public health expert at Yale University.

"This was the best hypothesis for an optimal delay before the second dose, in order to increase immunity," he told AFP.

"Small changes to the recommendations can make all the difference in giving the vaccines we have a much greater reach."

According to him, delaying the second dose - considered crucial for long-term protection - should only be done for people under 65 and without specific risks.

The United States had set the number of people vaccinated at 20 million by the end of December, but as of January 4, only 4.8 million people had received a first injection.

The country, like the UK, has covered around 1.4% of its population.

Europe is far behind, while Israel is at 13.5%.

- Last resort -

The vaccines from Pfizer / BioNTech and Moderna are approximately 95% effective after the second dose.

Moderna's vaccine, in particular, showed a high level of protection after the first dose - around 90% - a figure that should be treated with caution, however, as it is based on a small sample.

According to Saad Omer, director of the Yale Institute of Health, a change in strategy is only justified for countries where supplies are tight.

In the United States, where 17 million doses have already been distributed to the states, administering these already available doses is the priority, he told AFP.

Regarding injecting a different vaccine from dose to dose, for immunologist Akiko Iwasaki, it could work in theory, but experts agree that it requires more research and should not be done. as a last resort now.

One way forward could be to combine the results of several studies already carried out to determine which antibody threshold results in protection against Covid-19, according to Saad Omer and Natalie Dean, biostatistician at the University of Florida.

And then conduct smaller trials to determine what dose of vaccine gets to that level.

Such a study is underway to determine whether injecting half doses of Moderna's vaccine provides the same protection as full doses, according to The New York Times reported by John Mascola of the US National Institutes of Health. (NIH).

But one worry torments scientist Natalie Dean: that these changes are helping to fuel public mistrust of vaccines.

According to her, any modification should therefore follow the same authorization process as that used to authorize urgently vaccines.

© 2021 AFP