2.2 percent of NZ Covid-19 cases asymptomatic

Just a small percentage of New Zealand's confirmed Covid-19 cases have been asymptomatic, when overseas jurisdictions have seen far higher rates, Marc Daalder reports

Asymptomatic Covid-19 is a problem that haunts epidemiologists. People are more likely to go out if they're feeling fine, but it has become increasingly likely that some people with Covid-19 will never develop symptoms, while still being contagious.

Internationally, the percentage of asymptomatic cases varies widely, from as low as 10 percent to more than 50 percent of the infected population. This Newsroom explainer examines the research about how widespread Covid-19 - and asymptomatic Covid-19 in particular - really is.

Now, the first data on this crucial issue for New Zealand is available. Of the 1,138 people who have tested positive for Covid-19 in New Zealand, just 25 of them were asymptomatic, a Ministry of Health spokesperson told Newsroom.

Newsroom is seeking clarification as to whether this figure includes people who were asymptomatic when tested but who may have developed symptoms later or is restricted to just those who were asymptomatic for the entire period of infection.

Either way, the proportion of asymptomatic cases - just 2.2 percent - is far lower than in other jurisdictions. Why is this?

There are three possibilities.

The first is that, for whatever reason, New Zealand's experience isn't representative of how the virus actually operates. Perhaps the sample size is just too small, perhaps the fact that our infections have skewed younger is throwing off the data.

Second, our low proportion of asymptomatic cases could indicate that we're missing many more asymptomatic cases out there. This is, however, unlikely, infectious diseases expert David Murdoch told Newsroom. Murdoch is the Dean of the University of Otago, Christchurch, the co-leader of the Infection Group and a Senior Associate in the Department of International Health at Johns Hopkins School of Public Health.

When New Zealand has searched for asymptomatic infections, it has found none. More than 1,000 tests carried out at supermarkets in Covid-19 hotspots - Queenstown, Waikato, Canterbury and Auckland - all returned negative results.

Our positivity rate is another indicator. For every day in the past month, fewer than two percent of tests returned a positive result. In fact, the positivity rate was under one percent for every day but one in the past month.

This means despite widespread testing - generally more than 4,000, or 0.1 percent of the population, each day - we are still finding very few cases, indicating that we aren't likely to be missing many.

For comparison, between 10 and 17 percent of tests in the past week in the United States have returned positive results.

Given all of this, why is our asymptomatic rate so low? Murdoch says the third possibility is the most likely: Maybe everyone else has got it wrong.

"Looking through the expanding literature, seemingly, a large proportion of the cases that were called asymptomatic either, when they looked back, did have symptoms or subsequently developed symptoms," he said.

New Zealand has fewer cases and more resources to exhaustively investigate each one after the fact, while other countries may struggle to do so in the midst of an epidemic.

"It depends how hard people go back. I assume here, with any positive, they'll just get searched up the wazoo. In terms of the investigation of each case, did they have symptoms, who could they be in contact with, where it might have come from. So when you do that deep questioning, you'll get better results," Murdoch told Newsroom.

"If there's a manageable number, you're going to get better data. There's absolutely no question about that. I would think a lot of data out of the US, just because of the sheer number of cases, it just can't be as reliable."

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