Why is AstraZeneca not used as Covid booster? Why Pfizer and Moderna are used - and can you get AstraZeneca?

More than 37 million Covid booster doses have been administered across the UK as part of efforts to tackle the spread of the Omicron variant.

Findings show that around six months after a second dose of any of the coronavirus vaccines, protection against death with Omicron was around 60% in those aged 50 and over. But this figue increased to around 95% two weeks after receiving a booster dose.

Boosters have also been found to provide high levels of protection against hospitalisation from Covid-19.

After a Pfizer booster, vaccine effectiveness against hospitalisation after a primary vaccination course started at around 90%, dropping to around 75% after 10 to 14 weeks.

Moderna was found to offer a similar level of protection, with booster effectiveness against hospitalisation was 90% to 95% up to nine weeks after vaccination.

Findings from the CovBoost study, which assessed the effects of third doses on the body’s T cell immune response after a booster shot, back up the UK’s decision to offer either the Pfizer or Moderna vaccines as a third dose, with the mRNA jabs giving the most significant rise in immunity levels by comparison to other vaccines.

Professor Saul Faust, trial lead and director of the NIHR Clinical Research Facility at University Hospital Southampton NHS Foundation Trust, said: “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.”

Will I get the AstraZeneca vaccine as a booster?

The Pfizer, Moderna and AstraZeneca vaccines have all been approved for use as a booster by the Medicines and Healthcare products Regulatory Agency (MHRA).

However, the Joint Committee on Vaccination and Immunisation (JCVI) has recommended only using the two mRNA vaccines (Pfizer and Moderna) where possible, as these have been proven to have fewer side effects and a stronger immune response.

Those eligible for the booster will be offered either a full dose of the Pfizer vaccine, or a half dose of the Moderna vaccine, regardless of which jab individuals had for their first two doses.

In cases when neither of these vaccines can be offered, due to an allergy for example, the JCVI advises that the AstraZeneca vaccine can be used for those who received this jab for their first and second dose.

Is the AstraZeneca vaccine safe?

The MHRA advised against giving the AstraZeneca vaccine to people under the age of 40 earlier this year due to its link to rare blood clots.

People in this age group are instead being offered the Pfizer or Moderna vaccines as the balance of risk is “more finely balanced” among younger age groups, who do not tend to suffer serious illness from Covid-19.

Separately, a review by the European Medicines Agency (EMA) concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the AstraZeneca vaccine.

However, scientists believe they have identified the “trigger” behind the extremely rare blood clot complications from the jab.

An international team of researchers in Cardiff and the US have said the reaction can be traced to the way a component of the vaccine binds with a specific protein in the blood.

Researchers believe this may cause a chain reaction in the immune system which can lead to the development of blood clots, a condition known as vaccine-induced immune thrombotic thrombocytopenia (VITT).

Professor Alan Parker, from Cardiff University’s School of Medicine, said: “VITT only happens in extremely rare cases because a chain of complex events needs to take place to trigger this ultra-rare side effect.

“Our data confirms PF4 can bind to adenoviruses, an important step in unravelling the mechanism underlying VITT. Establishing a mechanism could help to prevent and treat this disorder.

“We hope our findings can be used to better understand the rare side effects of these new vaccines – and potentially to design new and improved vaccines to turn the tide on this global pandemic.”

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