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Cases of fully vaccinated Australians catching COVID-19 are part and parcel of how vaccines work and should not raise undue concerns.
Experts in the field say vaccines against COVID-19 dramatically cut a person’s risk of being infected and of getting seriously ill or dying but do not offer 100 per cent protection. However, if a vaccinated person does catch the virus, they are at dramatically lower risk of passing it on.
Staff at a mass vaccination centre at the Melbourne Exhibition Centre.Credit:Getty Images
In Sydney this week, a fully vaccinated nurse in Westmead Hospital’s COVID ward returned a positive test – a “breakthrough infection” – despite wearing full personal protective gear while at work. There are 132 people in hospital with COVID-19 across Australia, including 22 in intensive care.
In Melbourne, the virus got into a nursing home last month but because staff and residents were vaccinated, it barely spread and did not cause any deaths.
Compare that with the 45 deaths in the outbreak at St Basil’s Homes for the Aged in suburban Fawkner last year, before vaccines started rolling out.
Scientists say we should expect many more cases among vaccinated people but they shouldn’t seriously worry us.
“No, we shouldn’t be very concerned,” said Professor Caroline Miller, director of the Health Policy Centre at the South Australian Health and Medical Research Institute.
“The purpose of a vaccine is to prevent serious illness and death from a disease. That’s what all our vaccines do: the flu vaccine, the measles vaccine.
“What the vaccine does is it prepares your immune system to recognise the virus and overcome it more quickly and readily.”
Clinical trial data put Pfizer’s vaccine efficacy at 95 per cent and AstraZeneca’s at 81 per cent, while real-world data suggests both vaccines cut a person’s chances of falling sick by about 65 per cent.
Infections among vaccinated people attract media attention but avoided infections because of vaccines cannot be seen, said University of Sydney professor of medical microbiology James Triccas.
“We focus on what we see, whereas we’re not reporting the large majority who are vaccinated and not getting infected,” he said.
Professor Miller said a vaccine offering 100 per cent protection against a virus – known as “sterilising immunity” – was rare. Most vaccines offer lower but still important levels of protection.
When vaccinated people do get COVID-19, they are far less likely to show symptoms such as coughing and sneezing – which means they are less likely to spread the virus, she said.
Data from England suggests the Pfizer and AstraZeneca vaccines both cut the risk of passing on the virus by about 50 per cent – on top of the reduced chance of catching the virus in the first place.
In Sydney, the nurse at Westmead who became infected was reported to be asymptomatic.
“A mild breakthrough case is less likely to transmit and therefore you’re helping to control the spread,” Professor Triccas said.
Professor Miller said that what a vaccine that did not completely block transmission might mean to the future of the pandemic was “the million-dollar question”.
While virus transmission would be limited, current vaccines would allow COVID-19 to circulate at low levels in the community even if we reach full vaccination.
“We’ll be moving more into an endemic control phase. This virus is not going to disappear globally. We will inevitably have cases. Eventually, I think we will look at our targets being avoiding deaths, severe cases, hospitalisation,” Professor Miller said.
“It’s a virus that’s not going to disappear. And so it’s a virus we will have to find a way to control, as we do lots of communicable diseases.”
Liam Mannix’s Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.
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