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What are ‘breakthrough infections’ and do they actually show vaccines don’t work?

Katherine Hignett
Health Studio
United Kingdom

Reports of “breakthrough infections”, whereby someone who’s fully vaccinated still catches Covid-19, have appeared in the news in recent weeks. Scientists say these cases, which are relatively rare and often mild, are to be expected. But some social media users and anti-vaxx pundits claim they show the jabs aren’t working.

So, what’s the truth? Health Studio recruited two vaccine experts to separate the facts from the fiction.


What are “breakthrough infections”?


The term “breakthrough infection” refers to a SARS-CoV-2 infection in someone who is fully vaccinated. SARS-CoV-2 is the virus that causes Covid-19.

Being “fully vaccinated” means you’ve had all doses recommended for your particular vaccine, and that the shots have had time to take effect. That means you’ve had two doses of vaccines like Oxford-AstraZeneca, Pfizer-BioNTech, Moderna or Sinopharm jabs, or one dose of the Johnson & Johnson vaccine.

It takes time for your immune system to build protection following vaccination. So you’re not considered “fully vaccinated” until two weeks after your last dose, Martin Michaelis, a professor of molecular medicine at the University of Kent in the UK, told Health Studio.

“Notably, every infection in fully vaccinated individuals is registered as a breakthrough infection, independently of how mild or severe it is,” he added. “This means that even if a fully vaccinated person tests positive for Covid-19 without showing any symptoms, this will be regarded as a breakthrough infection.”


Why are some vaccinated people still getting infected?


While scientific evidence shows most authorised Covid-19 vaccines are highly effective against symptomatic disease, none of them offer 100% protection against infection. This means that some individuals will still catch Covid-19 even if they have had the jab.

Several factors, including waning immune response to vaccines over time and already having a compromised immune system, may make breakthrough infections more likely, explained Dr Kelly McKee, a chief medical officer at Revive Therapeutics and a vaccine expert with more than 30 years’ experience.

But this doesn’t mean the jabs aren’t working, in spite of what some online conspiracy theorists might claim.

“[Breakthrough infections] only show that the protection provided by vaccines is not perfect,” Prof. Michaelis said. “The risk of developing any Covid-19 symptoms is much lower in fully vaccinated individuals than in unvaccinated individuals, and the protection against severe disease is even higher.

“This is a little bit like safety belts and airbags in our cars. They save many lives, but they cannot save every single life.”


Do vaccines offer protection against new variants?


As SARS-CoV-2 has spread around the world, small changes have occurred in some of the infecting viruses, creating new variants. “Like most viruses, [SARS-CoV-2] evolves to maximise its chances to survive in the face of immune pressure from its host,” Dr McKee explains.

Some of these variants contain changes that help the virus evade the immune response generated by current vaccines, Dr McKee said, giving them “a competitive advantage over earlier, more ancestral, strains.”

This makes it easier for them to infect people who have already been vaccinated. Scientific evidence shows that the existing vaccines are slightly less effective against some new variants than the original version of the virus they were developed to target. As these variants replace older ones, more breakthrough infections are likely to occur.

The existing vaccines offer somewhat lower protection against the beta variant first discovered in South Africa, the gamma variant detected in Brazil and the Delta variant first found in India, compared to the alpha variant discovered in Kent and the original variant seen in Wuhan, China, at the start of the pandemic.

“Around the world, we see more and more variants that seem to have evolved partly to avoid immune protection provided by vaccines or previous infections,” Prof Michaelis said.

Giving booster shots six months or more after initial vaccination, as well as updating vaccines to target new variants, will improve the protection they offer, he added. But there will always be a delay between the emergence of new variants and the development of new vaccines.

Getting more people jabbed may help reduce transmission of Covid-19, although it’s not yet clear by exactly how much. This would mean fewer people, vaccinated or not, develop Covid-19 in a population, potentially giving the virus less opportunity to adapt.

Lower transmission also means there’s a lower risk of infection for everyone in a community: something particularly important for people who either cannot get vaccinated, or for whom vaccines aren’t as effective.

Prof Michaelis explained: “Low levels of virus spread are important to protect vulnerable individuals, whose immune systems are not fully functioning and who cannot be protected by vaccines in the same way as the rest of the population.”

Katherine Hignett -
Health Studio
United Kingdom

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