LOCKDOWN SCIENCE EXPLAINED

Prime Minister Jacinda Ardern says there is no plan for New Zealand to lift the alert level four lockdown early – and there is science informing that decision.

Because of the time lag it took for Covid-19 to rear its head, four weeks was the minimum time needed to ensure the chain of transmission was stopped, she said.

The virus could take up to 14 days to show signs, so cases being seen now could have had the virus prior to the lockdown, but were then asymptomatic.

The lockdown ought to have stopped wider transmission but those people may have passed the virus to close contacts before level four was put in place.

‘‘So we can expect to see these close contacts coming through now and next week as well. And of course those people could have passed it onto others in their bubble or their essential workplace as well.’’

Health officials needed to better understand the cases of community transmission and have certainty there was not a wider presence in the community than what was already known – especially in areas with low case numbers – before lockdown could lift, she said.

‘‘Absence of evidence is not evidence of absence. We need to be absolutely sure we are not missing a silent outbreak.’’

That was why surveillance testing would be so important and this would get under way soon, Ardern said.

This would be a version of sentinel surveillance such as that used to ascertain how the flu spreads.

It would be undertaken in regions where there was no ‘‘rich