Fact file

Which Covid vaccine works best as a booster jab?

Study reveals third Pfizer dose ‘offers near total protection’ against virus

A man receives his first Covid-19 dose in Surrey

A third dose of the Pfizer Covid-19 vaccine provides “excellent” immunity against the virus, according to the first full trial of the jab’s booster efficacy.

The pharmaceutical giant found that a booster shot of the vaccine is 95.6% more effective than two shots and a placebo at preventing infection. A researcher involved in the trial told the Financial Times (FT) that it represents a “big step forwards”.

The results come as Boris Johnson “suggested older people should be able to get boosters sooner than currently allowed”, The Times said. His intervention has “piled pressure on vaccine chiefs” to shift course and bring the date for boosters forward.

Research is currently ongoing into which vaccine is the most effective as countries around the world begin considering whether to launch booster campaigns.

Pfizer booster

The study of 10,000 people found that those who received a third dose of the Pfizer vaccine almost a year after their first two “saw protection against symptomatic infection soar compared with those who had had just two doses”, The Times said.

The research, which revealed that the booster jab “offers near total protection”, was the first randomised trial to “systematically” examine “the real-world protection offered by a booster”, the paper added.

Unlike previous trials its comparison group was not unvaccinated. Instead, it compared those who received three doses with patients receiving two jabs and a placebo. Five infections were registered in patients receiving the booster, compared with 109 who were given a placebo.

Ugur Sahin, CEO of BioNTech, the company that developed the vaccine, told the FT that the “important data” showed that “booster vaccinations could play an important role in sustaining pandemic containment and a return to normalcy”.

Andy Hill, a senior visiting research fellow in pharmacology at the University of Liverpool, added that the trial was “very important” as it provided a “much stronger” level of evidence.

The study follows the Cov-Boost trial which the UK’s Joint Committee on Vaccination and Immunisation said “indicated that the Pfizer vaccine is well-tolerated as a third dose among patients and provides a strong booster response”, The i news site said.

AstraZeneca booster

AstraZeneca vaccines will not be used during the UK’s booster campaign, though “the few trials to have tested extra doses” suggest that the jab “prompted a spike in levels of infection-blocking ‘neutralizing’ antibodies when administered several months after the second dose”, according to Nature.

Some concern still exists over the side-effects of the vaccine, with research ongoing into the links between the jab and rare blood clotting. 

An Oxford University study published in late August found that the chances of developing dangerous blood clots after being infected with the virus that causes Covid far outweigh the risks of the AZ vaccine. Though that small risk may still have informed the JCVI’s recommendation.

Moderna booster

While the government has said the UK will use the Pfizer vaccine for its booster campaign, the JCVI also said that the Moderna vaccine could be used as a third jab.

The drug company has released preliminary phase two trial results, which showed that the jab provided a “higher neutralizing antibody titers” against the Delta variant when administered as a booster, adding at the time that “dose three boosters will likely be necessary” for those who received the Moderna vaccine the first time around.

CNBC reported at the time that Moderna has signed deals to produce “800 million to one billion Covid vaccine doses this year”, as well as further “contracts worth $20bn [£14.5bn] in sales this year and agreements worth $12bn [£8.6bn] in 2022”.

Johnson & Johnson booster

The vaccine produced by pharmaceutical giant Johnson & Johnson is unusual in that it is a single-shot jab. The company is currently running a trial “to determine if the vaccine is safe to use and can induce an immune response at two dose levels”, said News Medical in August.

Preliminary results from that trial suggest “the booster shot elicited a rapid and robust increase in spike-binding antibodies”, the site added, with researchers suggesting that the response was “nine-fold higher than 28 days after the first dose”.

The trial included just 25 adults between the age of 18 and 55 at a single site in Boston, with an additional follow-up with the participants currently under way. The same trial has also been performed at clinical sites in Belgium, with results expected shortly.

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