Over the last few days in Victoria, there have been low numbers of new COVID cases - but every new case is not the same.

Recently, many of them are people already in quarantine, which shows that contact tracers have successfully found and isolated people before they can infect more people in the community.

But others, like two cases to be announced today, are slightly more problematic: they're another health care worker and aged-care resident.

And then there's the cluster that no one saw coming: the discovery of a different variant not known to be circulating in the community.

So on today's Coronacast, how's it looking for Victoria's hopeful reopening this Thursday?

Also on today's show:

* If the vaccines only stop severe illness and do not stop spread, how is herd immunity possible?

* Are the Delta and Kappa variants covered by the AstraZeneca and Pfizer vaccines?

* I've been donating plasma, but now that I'm vaccinated I'm feeling a bit selfish about giving away my newly made antibodies. Should I be concerned?