Covid reinfection rises

THE number of people being infected with Omicron after previously contracting a different variant of coronavirus is on the rise, as experts warn of limited immunity from the fastmoving strain.

Barwon Health infectious disease specialist Daniel O’Brien said there was still a lot unknown about reinfection, with a number of studies looking into this area.

“People who test positive to Covid a second time are likely to have been reinfected with a different strain of the virus,” Associate Professor O’Brien said.

“Reinfection is more common with the rise of the Omicron variant, as it can readily infect people who have tested positive to a previous strain such as Delta or Kappa.”

A new study, Report 49, from the Imperial College of London Covid-19 response team, indicated the risk of reinfection with Omicron after contracting a previous variant was 5.4 times greater than that of the Delta variant.

It also indicated that protection against Omicron following other infection was reduced to 19 per cent.

“Prior to Omicron, the SIREN cohort study of UK healthcare workers estimated that SARS-CoV-2 infection gave 85 per cent protection against reinfection over six months (16), or a relative risk of infection of 0.15 compared with those with no prior infection,” the report reads.

Deakin University epidemiology chairwoman Catherine Bennett said Omicron was a new challenge for the health system.

“Previously (a prior infection) really did reduce the risk,” Professor Bennett said.

“With Omicron, a prior infection, that is non-Omicron, only gives us a 19 per cent reduced chance of infection.

“So instead of being 85 per cent less likely to be infected, you have an 80 per cent chance (of being infected).”

She said early studies indicated that efficacy of two doses of a Covid-19 vaccine also reduced to about 19 per cent with Omicron, but a booster dose elevated the protection to greater than 60 per cent.

“It means whether you’ve had a prior infection or a vaccination at the moment Omicron has escaped the prior immunity from either past disease or vaccine at the moment,” she said.

“The difference is, with a vaccine we can have a booster.”

Prof Bennett said nationally the booster shots would likely reduce infection rates by twothirds.

“It makes a big difference,” she said.

Prof Bennett said the Omicron strain, which emerged in South Africa, was first noticed because people who were vaccinated or who had had the Delta strain were becoming infected.

But she said the current surge of virus would likely peak at the end of January to mid-February.

“We are not going to see it in one hit Australia-wide because the virus is moving into different communities,” she said.

“But we are actually seeing something that is starting to flatten out in those original hotspots.”

Health Minister Martin Foley said on Monday he also expected Omicron case numbers to peak in late January or early February.

“Then, of course, the delayed measures of hospitalisations, ICUs, ventilators . . . sadly this could lag for a week or two after that,” Mr Foley said on Monday.