Covid-19: how effective is a third dose of vaccine?

Another question is whether the immunity we get from booster programs could last longer if they were designed with updated versions of the vaccines, such as those that can recognize newer variants like Delta.

“You know, again, you could write a whole doctorate on this stuff,” says Altmann, “You have a complex world, you have all these different people and all these different models of protective immunity.” For starters, we were all infected and uninfected people – but the paths diverge quickly, making vaccine development more complicated.

Altmann explains that a person walking down the street may have been infected with the original variant of Covid-19, then received two doses of the Pfizer vaccine … then was re-infected with the Delta strain. During this time, another person may have received a different brand of vaccine and been infected with a different variant, such as the Alpha strain first identified in Kent.

“So how am I going to guess which sequence of spikes or which mutations to put in my vaccine to create something that might be useful for the next year or two?” Said Altmann. “It’s unbelievably not as simple as saying ‘what’s the last variation you last heard of, skip this one’ – it’s not that simple.”

In an ideal world, Altmann would like to see governments take into account this variability in a population’s immunity – and maybe even test people’s antibody levels to see if they really need a third dose. .

Other lingering questions include the size of the third dose – the UK plans to experiment with giving people less vaccine on the booster, to make more doses available to developing countries – and how many time after second it should be administered. For example, while Israel ran with a five-month gap, the United States plans to wait eight months between them.

A future booster

Perhaps the most pressing question of all, however, is how long a person’s newly restored immunity will last after their booster injection – and here at least there is a strong suspicion of what’s to come.

“I don’t think we know that,” Humar said. “But I think that’s a very real possibility. That’s what it’s starting to look like, especially as we’re seeing the decrease in antibodies and the decrease in efficacy against the variants.”

Altmann could also see booster programs eventually evolving into annual programs, but he’s not sure if science would back him up. “I think it would be a managerial logistical decision, not an immunological decision,” he says, explaining that it seems unlikely that the immunity of the population – with all the antibodies and T cells involved – will be low enough. to justify it.

“But I could easily imagine the health ministry planners saying, ‘well, the easiest thing is to do it at the same time every fall. So let’s just inject more doses into the world and do it, “” Altmann said.

It seems that Israel is already close to such a scenario. No sooner had they announced that they would provide third doses to all eligible citizens, when the country’s “virus czar”, Salman Zarka, suggested that they should start preparing for the fourth injections.

Zarka told The Times of Israel: “It’s our life from now on, in waves.

Zaria Gorvett is a senior reporter for BBC Future and tweets @ZariaGorvett

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