Just when you think you've got coronavirus pinned down, it manages to struggle free and throw a few mystery moves in your direction.

Late yesterday, NSW Health sent out an alert revealing two people from Sydney's Northern Beaches had tested positive to COVID-19.

There are currently no known links, which is a worry as it's been more than a month since authorities have been unable to trace the source of an infection in the community.

And if that wasn't enough, a Sydney van driver who was responsible for driving international flight crews to and from the airport had tested positive COVID.

So on today's Coronacast, how worried should we be?

Also on today's show:

* If a vaccine is only 90 per cent effective at stopping disease, but not spread, then how does it differ from having no vaccine?

* How long will it take to have everyone moving freely around again after being vaccinated, for instance if 100 people are vaccinated or 1 million people are vaccinated what's the number?

And we have an announcement about the summer break! Will Coronacast be back in 2021?