People may become reinfected with Covid-19 as early as 28 days after recovering from the virus, according to the latest advice from the Australian Health Protection Principal Committee. Previously, reinfection was defined as a case occurring more than 12 weeks after an initial infection.
The committee said the Omicron subvariants BA.4 and BA.5 “are associated with increased immune escape and we are likely to see rates of reinfection rise among those who have previously been infected with an earlier Covid-19 variant, and those who are up to date with their vaccinations”.
The BA.4 and BA.5 strains are becoming dominant throughout Australia, and have already been confirmed dominant in some states including New South Wales and Victoria.
What does this mean for testing and isolation requirements?
On Monday Western Australia’s chief health officer, Andy Robertson, said people who have had Covid-19 should again get tested and isolate if they have symptoms more than 28 days after recovering. A spokesman for the Western Australia health department said in the four weeks to 10 July, approximately 2,545 reinfections had occurred. The majority of cases are still new infections.
Other states including NSW and Victoria have since issued similar advice. If a positive test is returned after 28 days, this will be treated as a new case.
It is important to regularly check in with your state or territory health department for the latest advice. South Australia is still considering what rule changes might mean for isolation periods.
The NSW chief health officer, Dr Kerry Chant, said: “We’re urging people who have recently had Covid-19, even if they left isolation in the past four weeks, not to be complacent. If you develop symptoms again, make sure to test and isolate.”
Why isn’t a booster recommended after 28 days then?
The interval recommended between a Covid-19 infection and a booster dose is still three months. Anyone aged 30 or above is now eligible to receive their fourth vaccine dose if three months have passed from their third dose, or from being infected with Covid – whichever was more recent.
Professor and chair of epidemiology at Deakin University, Catherine Bennett, said most people won’t have an infection between their third and fourth booster doses.
“Atagi [Australian Technical Advisory Group on Immunisation] are mostly concerned with vaccine effectiveness in relation to the gap between doses, not the interruptions with actual infection,” she said.
“Most people won’t have an infection between doses and so Atagi are aiming for the best gap. They are balancing the best spacing between doses with the reality of rising exposure risk.”
Atagi frequently reviews evidence and may change its advice as the virus spreads.
Cassandra Berry, a professor in viral immunology with Murdoch University, said Atagi are using the rationale that either a booster or a natural virus infection with Covid-19 will provide greater protection from severe disease, hospitalisations and death. Receiving a booster 28 days after infection will not stop you from being reinfected, though it will decrease the risk.
Berry said the main aim for health departments revising the reinfection period is “to keep recently infected people alert to test for reinfection if they develop symptoms as soon as 28 days post Covid”.
“The point is these people shouldn’t go about their daily lives expecting to be totally immune,” she said.
“The aim here is to isolate the sick from the healthy in the community as soon as possible to slow the virus spread to others during this wave,” especially the vulnerable.
A spokesperson for the federal health department said individuals who have previously been infected with Covid, irrespective of which variant it may have been, should complete their vaccination course.
“Vaccination in addition to infection, as compared with prior infection alone, offers the best available protection against reinfection,” he said.
“The AHPPC advice relates specifically to the possibility of reinfection from 28 days post prior infection, it does not suggest Covid-19 vaccination would be advisable from this time.”
The vulnerable are most at risk of faster reinfection
Virologist and associate professor at the Kirby Institute, Stuart Turville, said those most likely to be reinfected after just four weeks are those with weakened immune systems.
“Those that had a poor immune response to their initial infection and vaccination are the ones that now have been reinfected,” he said.
“The good news though is those with initial robust responses to the virus, back in March 2020, and then vaccinated have not been reinfected so far after two big Omicron waves. Those people have had in theory four immune encounters, with an infection and then three vaccine doses.”
He said expanded access to a fourth vaccine dose “will help level the immune playing field,” because there may be some people who are immunosuppressed and who don’t respond well to two or three vaccine doses, but who don’t realise that they have underlying conditions and are at risk.
