An epidemiologist is warning a summer wave of Covid-19 is approaching, which will see an increase in infections and deaths.

It comes as cases of a new highly transmissible subvariant XEC have been reported in New Zealand this month.

While University of Otago professor Michael Baker told Sunday Morning it is easier to reduce transmission in summer, summer waves can still be very harmful.

"The trouble with these waves isn't so much that the new subvariants that are causing them are more harmful, it's mainly that they just infect a whole lot more people," he said.

"The number of deaths might increase from, say, five a week up to maybe 30 or 35 per week. And we're going to get more cases of long Covid."

Baker said people had become "very complacent", and while the Covid-19 pandemic had officially ended in May 2023, there are still two waves - a winter and a summer one - that can cause a lot of harm.

"We all want this to go away. I mean, it's been very disruptive and we can't wish it away," he said.

Baker wanted more people to have access to booster shots.

"I think we should lower that age of eligibility," he said.

"We don't allow people under 30 years of age to get any boosters. And I think that's a mistake because they can get long Covid. They need protection."

Baker added the booster shots were continuing to provide reasonable protection against infection, hospitalisation, death, long Covid - as well as improve outcomes in pregnancy - even with the new XEC subvariant.

Long Covid was probably the biggest health impact of the virus, Baker said.

He said around 6 or 7 per cent of adults across the globe are now affected "significantly".

"It's a complex, multi-system disease. It can affect almost every organ in the body. It can be severely disabling."

The economic cost is estimated at about 1 percent of the global economy, Baker added.

He warned repeat infections increase the risk of long Covid too.

"I think that's a major source of concern, even if the infections actually appear to be quite mild, even if they're asymptomatic."

He also warned there was a "small signal of increasing resistance" against antivirals.

"One of the things about microorganisms is they're constantly evolving and they almost inevitably get a degree of antimicrobial resistance over time.

"There is a signal coming through with the main component of Paxlovid, for instance, and also Remdesivir, which is used for people in hospitals, that there is a small signal of increasing resistance.

"It's not clinically important yet, but it is a warning on the horizon that we need to manage the use of these antivirals carefully."

RNZ