Fact file

Which Covid-19 vaccine works best as a booster jab?

Moderna vaccine found to raise antibody levels up to 32-fold

A man receives his first Covid-19 dose in Surrey

The Moderna and Pfizer vaccines have been found to be the most “potent” booster jabs, according to a UK study examining seven different vaccines.

The trial, published in the medical journal The Lancet, found that a full dose of the Moderna vaccine - currently given as a half dose in the UK vaccination programme - and the Pfizer vaccine are “highly effective boosters”, although scientists have “cautioned about comparing their performance as people started with different antibody levels”, said The Guardian.

The two vaccines make up the lion’s share of the UK government’s booster programme. The results of the study suggest ministers were “justified” in their decision to select the two vaccines as booster jabs, said the BBC

The study involved almost 3,000 people aged 30 and over, and found that mRNA type vaccines, such as Moderna and Pfizer, “gave the best boost to antibodies and T-cells”, particularly after two initial doses of the AstraZeneca jab, reported Sky News.  Both antibodies and T-cells are important factors in how well vaccines work. 

The mRNA jabs also provided the best boosts overall, regardless of the type of jabs a patient may have initially received. The study found that the six vaccines studied - Pfizer, AstraZeneca, Janssen, Moderna, CureVac and Novavax - gave a “significant immune boost”, regardless of what was given as their initial jabs, The Telegraph reported. 

“All of the vaccines in our study do show a statistically significant boost... RNA (Pfizer and Moderna) very high, but very effective boosts from Novavax, Janssen, and AstraZeneca as well,” said Professor Saul Faust, trial lead and director of the NIHR Clinical Research Facility at University Hospital Southampton NHS Foundation Trust. 

The trial did not test vaccinations against the newly discovered Omicron variant, but the results of the study revealed “how widely booster jabs can rally the immune system, making scientists cautiously optimistic as the world prepares for a wave of Omicron cases”, said The Times.

Moderna booster

In the latest study, a full dose of the Moderna vaccine was found to raise antibody levels 32-fold in people who had been given two initial doses of AstraZeneca. Currently, when Moderna is used in the UK only half a dose is given. Antibodies were raised 11-fold in those who had initially received two doses of Pfizer.

Overall, it was found to be the booster that most dramatically strengthened the body’s immune defences. 

The results of the trial has prompted the UK to secure an extra 114 million doses of Pfizer and Moderna vaccines to be delivered over the next two years, reported the BBC.

In May, the drug company’s preliminary phase two trial results found 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”.

Moderna has also said it is working on a booster shot tailored to the new variant.

Pfizer booster

In the Lancet study, those who initially received two doses of AstraZeneca and then received a booster Pfizer jab had a 25-fold increase in antibodies. When the Pfizer booster was given following two Pfizer shots, antibody levels rose more than eightfold.

The findings also suggest that a half dose of Pfizer “could have an effective response as a booster” and could “help with vaccine supplies”, reported The Washington Post. The trial didn't test a half-dose of Moderna.

In October, the first full trial of the jab’s booster efficacy found that a third dose of the Pfizer Covid-19 vaccine provides “excellent” immunity against the virus.

Last month, 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 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.

The findings came before the recent discovery of the Omicron variant, but Danny Altmann, professor of Immunology at Imperial College London, said: “If you’ve got a glimmer of evidence, and I think we have, that the vaccines are still holding up to some extent, then it would be a good idea to get as many people boosted as quickly as possible to raise their level of protection.”

Pfizer has said that, if needed, it can ship a new vaccine tailored to the emerging variant in around 100 days.

AstraZeneca booster

The latest study suggests that although the AstraZeneca booster did increase immune responses in people who had previously received two initial doses of the AstraZeneca jab, it was not as pronounced as the Moderna and Pfizer booster jabs, raising antibody levels in “a threefold increase”, said The Times

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.

Nevertheless, the NHS said: “Some people may be offered a booster dose of the Oxford/AstraZeneca vaccine if they cannot have the Pfizer/BioNTech or Moderna vaccine.”

Johnson & Johnson booster

The vaccine produced by pharmaceutical giant Johnson & Johnson is unusual in that it is a single-shot jab. In the US, a second J&J shot has been approved as a booster.

Results of a trial published in September found that protection against moderate to severe disease in people given a second J&J vaccine between two and six months after their first rose from 85% to 94%. Their amount of neutralizing antibodies also increased four-fold, explained Maureen Ferran, associate professor of biology at Rochester Institute of Technology, for The Conversation.

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