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While the emergence of the omicron variant has caused concern worldwide, there is cause for some optimism: emerging evidence on vaccine booster programmes reveals that a third dose can take people’s coronavirus protection to unexpectedly high levels.

It has long been predicted that the covid-19 vaccines from Oxford/AstraZeneca and Pfizer/BioNTech, which were designed as two-dose regimens, may eventually require a third shot. After studies suggested that vaccine effectiveness was waning, many countries began booster programmes, including the UK, which began offering third doses in September to people who are 50 or older and certain other groups. It later widened that to those aged 40 and over.

There was disappointment that boosters were needed after only six months, but the initial signs for how well third jabs are working have been no let-down. In October, a randomised trial found that people who had received a third dose of the Pfizer/BioNTech vaccine had than people who had only had two jabs.

While vaccine effectiveness tends to be lower in the wider world than in trials, real world figures have also been encouraging. In people over the age of 50, those who had a booster were about than those who were unvaccinated, regardless of whether their first two jabs were AstraZeneca or Pfizer/BioNTech, according to an analysis by the UK Health Security Agency (UKHSA). “It’s really impressive,” says at the University of East Anglia, UK.

The most recent results from the UKHSA suggest that, in the over 70s, for example, protection levels are now higher than they were in August, and seem to be continuing to rise.

People such as , the White House chief medical adviser, have said the covid-19 vaccines should be seen as requiring a three-dose regimen as standard. “A booster isn’t an add-on – a booster is part of what the original regimen should be,” .

Several other vaccines require three doses, such as the one against the liver infection hepatitis B. Giving sequential doses takes advantage of the fact that when we repeatedly encounter a pathogen or vaccine, our antibody-making cells undergo a process called “affinity maturation”.

Our antibodies are made by immune cells called B-cells, and during affinity maturation, these multiply within the body’s lymph nodes while undergoing mutations. Only the B-cells that make the best antibodies survive and replicate, so as a result, their progeny make ever-stronger antibodies. “With other infections, the third booster protects you for longer and also gives you antibodies that have higher affinity,” says Hunter.

This is good news for the many countries rolling out booster programmes. But information on the effectiveness of a third dose all relates to the delta variant, so how does the emergence of omicron change things?

Much about omicron is still unclear, but if it does replace delta as the dominant covid-19 variant, its genetic sequence suggests that the existing covid-19 vaccines may be less effective. That doesn’t mean vaccines will become useless, though. “Vaccination is still likely to protect you from severe disease,” , a member of the UK government’s Scientific Advisory Group for Emergencies, told the BBC on 27 November.

That means encouraging as many people as possible to take the full three-dose course of vaccines is still a good plan of action for tackling any covid-19 variant, whether delta or omicron.
On 29 November, the UK’s Joint Committee on Vaccination and Immunisation said that anyone 18 and over can have a booster, in order of descending age groups, and the required interval between second dose and third has shrunk from six months to three. The same day, the US Centers for Disease Control and Prevention said everyone aged 18 and older should get a booster too.

“If you’re boosted, your response is likely to be stronger so it’s more vital than ever that people get their jabs and we get those boosters into arms as fast as possible,” said UK prime minister Boris Johnson on 27 November.