Why is it taking so long to get to zero?

New Zealand has had no new cases of coronavirus for 13 days, but we’re not – quite – at zero cases of the disease.

For almost a week, we have had just a single active case of Covid-19 reported in the country.

The patient – a person aged in their 50s – is in the Auckland region, and their illness is linked to a rest home cluster.

The Ministry of Health has not released further details, but information on its website suggests the person could be a woman whose infection was first reported on May 1.

If correct, that would mean her case has been active for more than a month.

Many patients report a much faster recovery, but a long ‘‘tail effect’’ can occur in complicated cases of the infection.

Some Covid-19 patients can be unwell for up to two months, researchers told Britain’s Sky News last month.

Tim Spector, professor of genetic epidemiology at Kings College, London said about one in 20 Covid-19 patients were unwell for more than a month, and some for two.

‘‘You can have people who just have a rash on their toe that lasts a day and then goes away and that’s it,’’ he said.

‘‘Some people seem to be fine for a few days, then go back again. We don’t understand why and we need to urgently, actually, work out whether these people are still secreting the virus.

‘‘Is it that the virus is still in their system and they’re still reacting to it? Is it their immune system, or maybe they’ve developed – as in other viruses – some form of post-infective problem?

‘‘It’s unlike any disease I’ve studied in my 30-odd year career and I’ve looked at hundreds of different diseases.’’

Professor Kurt Krause, an infectious disease expert from the University of Otago, said that whether a person suffered a long tail effect was probably down to several factors.

Existing evidence and research showed it was relatively common for Covid-19 sufferers with major complications – such as strokes or coronary symptoms – to take months to recover, he said.

Kraus said a case being classed as ‘‘active’’ for more than a month did not surprise him.

‘‘Imagine that if you’ve had a severe assault on your lungs and your virus is cleared but you’re not breathing back to normal. They might be recovering from one of those side effects,’’ he said.