Covid 19 coronavirus: Half of those infected with virus might not even know it

Up to half of people infected with Covid-19 might not even know it.

Dr Michael Baker, epidemiologist and professor of public health at the University of Otago, says we should assume that half of those infected with the virus will show few or no symptoms.

"Some people have no symptoms at any time during their illness," he told the Herald.

"They get infected, they have evidence of being infectious but have no symptoms. We see this particularly in young people - you only know they have it because they've had a positive test.

"Then there are those that are pre-symptomatic, who are infectious for a few days before they get symptoms and can be infectious during that time."

People who are pre-symptomatic will display symptoms of the virus after they have tested positive for the coronavirus, Baker said.

As the lockdown lifts to level 3 from tomorrow morning, Kiwis have to remain vigilant in the fight against the virus, Baker said.

"It really is transmitted from person to person - you can't let your guard down or underestimate it at all.

Dr Michael Baker, epidemiologist and professor of public health at the University of Otago in Wellington. Photo / Mark Mitchell

"We are very good at testing and investigating virus cases in New Zealand. We have to find every case and stamp it out to achieve elimination," Baker said.

And elimination is what we've achieved, Prime Minister Jacinda Ardern said today.

However, added director general of health Dr Ashley Bloomfield, that doesn't mean no new virus cases will pop up. Rather, "we know where our cases are coming from".

Baker said modelling showed that six to eight weeks of high-level containment was needed to quash the spread of Covid-19, so now wasn't the time to ease prevention efforts.

"I am concerned that people will think they've got through it and can fully relax," he said.

"Everyone in the country understands social distancing and all of the other prevention methods. If you carry these things forward, we've got a good chance of beating the virus.

"Stay at home unless you've got a good reason to go out - everyone needs to keep in their heads the messages about transmission."

• Covid19.govt.nz: The Government's official Covid-19 advisory website

The number of daily new coronavirus cases has been in the single digits for eight days in a row, with an uptick in cases yesterday.

With 39 per cent of cases related to overseas travel, it remains the highest source of transmission in the country, ahead of cases related to the overseas infections, at 33 per cent.

"As we achieve elimination, we're still going to get people returning to New Zealand and people that might have symptoms or be new cases of coronavirus," Baker said.

"In between that, we will see cases that are ongoing chains of transmission which are what we expect to see and show how hard it can be to stamp out this infection."

Twenty-three per cent of New Zealand's cases are linked to locally acquired cases with a clear epidemiological link, while the source of 3 per cent of the locally acquired infections remains unknown.

Only 2 per cent of cases are under investigation, Ministry of Health data shows.

But cases linked to the 16 significant infection clusters across the country have continued to crop up.

"As we see with the rest home clusters, you can have quite long trains of transmission - that's why we know it's a tough virus. It's very infectious," Baker said.

A Ministry of Health spokesman said new cases of coronavirus could be linked to existing cases in myriad ways.

"Due to the incubation period, it may be some time before the cases emerge that are linked to clusters or confirmed cases," he said.

People may initially be classed as close contacts of a confirmed Covid-19 case, before becoming symptomatic for the virus.

While the pandemic was still in its infancy, the World Health Organisation identified household transmission to be the leading cause of secondary virus infections for the coronavirus, he said.

"This means that in household bubbles as well as in community settings, the core public health messages are vitally important: physical distancing, cough and sneeze etiquette, washing hands and more importantly being alert to signs of being unwell and acting appropriately if you are."

A number of factors can influence the daily tally of new Covid-19 cases, from when the tests were processed, to when the public health board was notified of the new cases, rather than when someone fell ill.

The Ministry of Health now includes probable cases of the virus in the daily count of confirmed cases as part of standard practice in infectious disease tracking, Baker said.

"In an epidemiological outbreak it's important to identify people who have symptoms and are a probable case of coronavirus," he said.

But globally, the number to keep an eye on is the number of deaths linked to the virus.

"Overseas they're not trying to identify every case as we are in New Zealand, so they're counting the number of deaths," he said.

"We have three million cases recorded globally, but that could be conservative - the true number of cases is probably 20 million.

"The virus is estimated to infect 50-60 per cent of the world in the next few years, it's just the nature of it. Many millions of people will die from it."

So much so that the global death rate could rise 50 per cent above the normal number in the next few years, Baker said.

In New Zealand, modelling predicted a 45 per cent excess of the yearly death rate - which is normally 33,000 a year - were the virus not contained.

"We're only at the beginning. Its going to be a major global problem for the next two years," Baker said.

"We have to do everything we can to stop the virus, because the alternative is so much worse."