Adults given AstraZeneca much more likely to have Covid ‘breakthrough’ infections

ADULTS given the AstraZeneca vaccine were much more likely to suffer “breakthrough” Covid infections compared to those given the Pfizer vaccine, researchers have found.

A study involving all fully vaccinated adults in Belgium showed substantially higher protection from mRNA vaccines such as Pfizer and Moderna compared to the AstraZeneca and Janssen brands, which used a more traditional viral vector formula.

People who had recovered from Covid infection prior to being vaccinated also had the lowest risk of breakthrough infection overall.

The findings are being presented at the conclusion, today, of the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal.

The results are also published in the journal, ‘Viruses’.

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Uptake was high in Belgium, with more than 80 per cent of adults having received two Covid vaccine doses by August 11 2021.

Belgium used four types of vaccine: Pfizer/BioNTech; Moderna; Oxford/AstraZeneca; and Janssen.

In the UK, the AstraZeneca vaccine was largely given to over-40s for the first two doses, with under-40s mainly allocated the Pfizer vaccine.


The study, led by Dr Veerle Stouten, an epidemiologist and expert in immunology at the Belgian public health institute Sciensano, studied the incidence of breakthrough infections – where the virus ‘breaks’ the protective barrier of vaccination – for each vaccine brand.

The study included more than eight million adults in Belgium who were fully vaccinated against Covid-19 between February 1 and December 5, 2021.

Participants were followed up for an average of 150 days, from 14 days after their second dose.

Nearly one in 20 (4.6%) had a breakthrough infection, which the researchers calculated would translate into a rate of 11.2 breakthrough infections for every 100 fully-vaccinated people over the course of a year.

The researchers found that the AstraZeneca vaccine was associated with a 68% higher risk of a breakthrough infection than the Pfizer vaccine after adjusting for variables including age, sex, prior Covid-19 infection, and a person’s exposure to the SARS-CoV-2 virus through their profession or environment.

Adults doubly-vaccinated with Moderna were also 32% less likely to experience breakthrough infection compared to those given two doses of Pfizer.

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There was a higher incidence of breakthrough infections in younger age groups (18-64 year olds) than in older age groups (65-84 or 85-plus year olds), which the scientists said might be due to differences in social behavior.

Healthcare workers were 40% less likely to develop a breakthrough infection than non-healthcare workers, probably reflecting high vaccine coverage and intensive use of PPE while at work.

The analysis also found that those with a prior Covid-19 infection before vaccination were 77% less likely to have a breakthrough infection than those who had never had Covid previously.

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The researchers stress that the study was not designed to carry out a formal comparison of vaccine effectiveness between brands, however.

They also noted that period of study largely covered the time when the Alpha, and then Delta, variants were dominant.

The Omicron strain has been associated with higher levels of breakthrough infection due to mutations which better-equip it to evade immunity built up from vaccination or prior infection.

In the week ending April 17 in Scotland, 3,242 of the confirmed Covid cases were reinfections (where a person tests positive more than 90 days after their last positive test) – equivalent to more than 10 per cent of the known cases that week.

HeraldScotland: Reinfections have become more common since Omicron emergedReinfections have become more common since Omicron emerged

The Belgian study also predicts effect of boosters.

In the UK, Pfizer – and to a lesser extent, Moderna – were used for third doses.

Dr Stouten said: “We identified risk factors associated with breakthrough infections, such as vaccination with adenoviral-vector vaccines, which could help inform future decisions on booster vaccination strategies internationally.

“Moreover, we observed that hybrid immunity of combined prior infection and vaccination not only lowered the risk of breakthrough infections but also of having symptoms when experiencing a breakthrough infection, highlighting its protective effect.

“The majority of the breakthrough infections included in the study occurred during the period when the Delta variant was dominant.

“We expect to see similar patterns regarding characteristics of breakthrough infections due to the Omicron variant but we need to continue to monitor breakthrough infections and study their severity and multiple recurrences, as well as the role of emerging variants to confirm this.”

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