Can you catch Covid-19 from someone without symptoms?

The recent confirmation of coronavirus cases in new Zealand is an an important reminder that we should be mindful of our behaviour, but there is no strong evidence of asymptomatic transmission, writes Associate Professor Siouxsie Wiles.

The Ministry of Health yesterday announced a second person in Aotearoa New Zealand has tested positive for the coronavirus that causes Covid-19*. Just like our first, this person has contracted the virus overseas. This time it was during a trip to Northern Italy. The woman, in her 30s, had travelled with her partner, who has been tested. At the time of writing his results were imminent.

*Since the time of writing, a third case has been confirmed.  

It wouldn’t surprise me if we end up with a few more confirmed cases connected with this latest one, as it sounds as though the person may have been out and about for the last few days even though they weren’t feeling all that great.

If we end up with an outbreak here, this is how it is probably going to happen - with us “soldiering on” when we should be staying home.

This announcement is a good reminder to us all to be mindful of our behaviour. If you have any symptoms of coughing, shortness of breath, and/or a fever, especially if you’ve been overseas recently, or been in close contact with someone who has been overseas, please stay home and call Healthline (0800 611 116) for advice rather than going about your usual business. And don’t turn up at your GP’s office or local hospital without calling ahead. I get that for some people, not going to work is financially impossible, so this is something we really need to sort out, and soon.

Asymptomatic transmission

There is currently a lot of confusion about whether this coronavirus can be spread by asymptomatic people – that is, people who are not displaying symptoms of illness. It can’t be ruled out for certain, but there doesn’t seem much strong evidence. The original paper that started this ball rolling turned out to be based on people assuming the original person didn’t have symptoms. Only it turns out they did.

Lots of people have been sending me links to news articles mentioning asymptomatic transmission but when I’ve read through them it is either noted that people had some mild symptoms, or there were clearly other opportunities for people to have been infected elsewhere.

While asymptomatic transmission is still a possibility, the information from the recent WHO China Joint Mission Report does show that the majority of people who have contracted Covid-19 have had close contact with someone with the virus and that person had symptoms.

There’s very clear data from China that people who don’t have symptoms are not ones who spread the virus. The virus is spread through droplets spread by coughing and sneezing.

Ashley Bloomfield Director-General of Health and Chief Executive, Ministry of Health

What is clear though is that even people with very mild symptoms can be infectious. The sensible approach remains: be vigilant, be prepared, but don’t panic.

Associate Professor Siouxsie Wiles is a microbiologist who knows a lot about bacteria, viruses and infectious diseases. She is based in the Department of Molecular Medicine and Pathology, located within the Faculty of Medical and Health Sciences' School of Medical Sciences.

This article reflects the opinion of the author and not necessarily the views of the University of Auckland.

Used with permission from The Spinoff, Can you catch Covid-19 from someone without symptoms? 5 March 2020.

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