COVID-19, flu boosters could be combined

"It is expected to work better than vaccines for the flu," Dr Balasubramaniam said on Tuesday. 

That's because coronavirus don't mutate as quickly as the flu. The flu vaccine is tweaked every year depending on the new dominant strain.

Dr Balasubramaniam says as COVID-19 enters its endemic phase, people are likely to need booster shots for both COVID-19 and flu viruses annually.  

"Depending on how much the coronavirus mutates and changes, it is highly possible it will be rolled into one," he said. 

Booster shots "significantly increase immunity" against the Delta strain of COVID-19, he said. 

In NSW, those who received their second dose six months or longer ago are now eligible for a Pfizer booster.

Boosters are expected to be available nationwide from Monday.