Coronavirus: Why fully vaccinated people get re-infected

Data from some countries with high vaccination rates are now showing more new cases of COVID-19 each week in vaccinated people than unvaccinated.

While these so-called "breakthrough infections" have raised questions of vaccine effectiveness, experts say this is to be expected, and not an indication that there are problems with the COVID vaccines.

"The issue is not the vaccines, it is the public's and politicians' misperceptions of what vaccines can and can't do that is the problem," says Professor Nikolai Petrovsky, from Flinders University, Adelaide.

To understand the point of vaccinations, it's important to understand how vaccines work.

Picture your immune system as a patrolling police officer and the virus as a criminal. Vaccines are like placing a "wanted" poster in your body: they tell the immune system what to keep an eye out for. So once a virus enters the body, the cops can recognise and arrest the criminal before they form their own little virus mafia - and commit crimes and in your body by replicating, which can lead to further virus spread, hospitalisation and even death.

While vaccines are not 100 per cent effective in preventing infection, Associate Professor Paul Griffin from the University of Queensland says that breakthrough infections "are likely to be much less severe than they would have been had the individual not been vaccinated and they are also less likely to pass it on to others".

And while it's disheartening to hear that confirmed COVID-19 cases in vaccinated people are on the rise, Dr Roger Lord from the Australian Catholic University says this is a simple numbers game and to be expected.

"It is not surprising that in countries where higher vaccination rates exist that more vaccinated than unvaccinated individuals have contracted COVID-19," he says.

"If for example, 80 per cent of a population is completely vaccinated and 20 per cent unvaccinated then statistically more vaccinated people may contract COVID-19."

Those vaccinated are also more likely to be out and about, adds Griffin, so it's logical they'll be more likely to come into contact with the virus.

"People who are fully vaccinated in some countries are also a little more likely to get infected as they are very rightly allowed more freedoms than their unvaccinated counterparts," he says.

"They also may have a reduction in their perceived risk and may then be less likely therefore to use other strategies to protect themselves such as social distancing, hand hygiene and wear[ing] masks, for example."

So while vaccination is important in reducing virus spread, hospitalisation and deaths in the community, it's still not a quick fix to get our lives back to normal - we will still be social distancing, hand washing and locking down for a while longer.

But this hard work is worthwhile, Griffin says, because a combination of our everyday activities and high vaccination rates can not only reduce the spread and severity of disease, but also the creation of new variants of concern, such as the Delta variant.

"For the mutations that result in new variants to occur, the virus has to be reproducing in a host, ie infected people," he says.

Essentially, the more efficient our tiny immune police are, the less time the 'criminal' virus will have to evade capture, so it won't have as much of a chance to mutate.

"Certainly, breakthrough infections means that the risk of new variants arising is likely to never be reduced to zero, but the more people vaccinated, the less opportunities for new variants to arise there will be," Griffin says.

And while the idea of vaccinating, hand-washing and social distancing feels never-ending, Curtin University's Professor Jaya Dantas points out that the key to stopping the pandemic is "for us to reach a high enough level of immunity that the virus has nowhere else to go".

Despite breakthrough infections, the experts say vaccines are working - reducing severe illness and hospitalisation.

And eventually, with enough measures in place and a high vaccination rate, Lord says lockdowns will be a thing of the past.

"Until that happens, individuals will need to continue to wear masks to help stem transmission of the virus widely in the community," he says.

But Dantas and other experts remain hopeful of a lockdown-free future.

"We can get there - but all of us have to do our part and get vaccinated."

You can read more of the Australian Science Media Centre's Expert Reaction to breakthrough infections here.