The nightmare of asymptomatic cases
Limited data from overseas is raising the possibility that large numbers of people are carrying the Sars-CoV-2 coronavirus without any symptoms, but community testing in New Zealand in recent days has turned up no similar asymptomatic cases here.
It’s thought people who never develop symptoms can still transmit the virus, and there’s also evidence that people who do end up showing symptoms are most infectious just when they start showing symptoms, or in the day or two beforehand.
That makes it much harder to trace and isolate everyone with the disease before they start transmitting it to others.
‘‘Asymptomatic cases are an absolute nightmare in terms of spread,’’ says Murray Cox, Massey University Professor in Computational Biology.
The community testing in New Zealand in recent days showed the virus was not ‘‘floating around the community at high levels’’.
That was good news, but not enough testing had been done to know whether the virus was in the community at low levels. If it was, that could be enough for Covid-19 to blow up again once restrictions were eased.
Health authorities carried out 343 community tests at a supermarket in Queenstown on Thursday, 308 in Waikato on Friday and 442 in Auckland on Saturday. None of those tests were positive.
The Ministry of Health has also reported that community testing in Canterbury had returned no positive results, as of Saturday.
At the daily briefing on Sunday, director-general of health Dr Ashley Bloomfield said the wider community testing was done to help confirm there was no underlying community transmission of Covid-19.
‘‘We have done a lot of testing in New Zealand, and particularly over these last two weeks, as we’ve seen the number of confirmed cases drop.’’
The vast majority of cases in the past three weeks could be linked to existing cases, overseas travel or existing clusters.
The community testing results were encouraging, ‘‘and that goes alongside the number of positive cases we’re seeing each day, with that wide testing right across the country and and across the range of ethnic groups as well’’, Bloomfield said.
Massey University’s Cox is surprised the community testing had found no-one displaying coronavirus without any symptoms. ‘‘But they aren’t picking up anyone, which is really good news,’’ he says.
At the same time, despite the lockdown, new cases of people with Covid-19 were being found, and it was not entirely clear where those were coming from. That led him to think the virus was floating around in the community.
It may be that not enough community testing has been done but it is possible the coronavirus in the community is at really low levels, Cox says.
Worldwide, it was unclear how many people who had the virus were asymptomatic. Some of the best data so far was from the US aircraft carrier Theodore Roosevelt.
In that case, US Defence Secretary Mark Esper said it was ‘‘disconcerting’’ to find about 60 per cent of those infected were not showing symptoms when tested. At the time he was speaking, testing of the 4800 crew members was mostly complete, with about 600 of those on board testing positive, and more than 350 of those being asymptomatic at the time of testing.
The 60 per cent figure was ‘‘absolutely huge’’, Cox said. He suspected it could come down as some crew members developed symptoms later.
Along with the data from the aircraft carrier, information from the small north Italian village of Vo` was also useful. In that case, Florence University professor of clinical immunology Sergio Romagnani reported on blanket testing in the isolated village of about 3000 people. All those with positive tests were quarantined, and the number of people sick from Covid-19 fell from 88 to seven in fewer than 10 days.
In an open letter to the authorities in the Tuscany, Romagnani said a research swab was performed on all the inhabitants of the village and it was shown between 50 and 75 per cent of those who became infected were completely asymptomatic.
Cox says some other information from overseas suggests the number of people who are asymptomatic could be between 25 and 50 per cent of cases.
It did seem many people could have the virus without showing any symptoms, while presumably being able to transmit it.
‘‘There’s very little testing out there. We probably won’t know the details until it’s all done and dusted but at the moment it does seem to be there are lots of asymptomatic cases.’’
That was different to standard influenza, where infected people tended to show symptoms.
‘‘That’s a really big problem . . . Asymptomatic cases are an absolute nightmare in terms of spread.’’
It means people in the community are spreading the virus through no fault of their own. ‘‘That means it’s very hard to stamp out.’’
It is also a problem for contact tracing, which works well when people know they are sick. But it can be hard to work out how people became sick if there are gaps in the middle where no-one knew they had the virus.
While the community testing in New Zealand so far is helpful, the virus could be in the community but at such low levels that the number of tests so far would not pick it up, Cox says. ‘‘It does tell us it’s not floating around the community at high levels . . . That’s really good news.’’
But the question of how many people have the virus has not been answered. ‘‘If you had some really low-level infection but then you open up everything and everyone goes back to work, then it might blow up again. In testing it’s always really hard to know with small numbers, say one or two in 100,000 people. That might be enough to re-infect the country.’’
It needs to be remembered that the disease is not expected to be very prevalent in this country, says Professor Shaun Hendy, director of a research centre at the University of Auckland focused on the study of complex systems and networks.
‘‘We haven’t had the sort of outbreak in the way Italy has, and we haven’t all been living on a ship.
‘‘We do think there are probably some asymptomatic cases out there but they would be very hard to find with random testing,’’ he says.
The random testing is a little bit of reassurance that the disease hasn’t got away on us.
It is hard to work out from the international data how many people remain asymptomatic and how many went on to develop systems after being tested.
‘‘There’s a few studies but it’s quite difficult, often they are a snapshot, often lacking information about who went on to develop symptoms.’’
Massey University professor in mathematical biology Mick Roberts says only a small number of people have been tested at random, compared to the population as a whole.
‘‘If you find people infected, then you know it’s out there. If you don’t find anyone infected, then you have to think, how hard did we look,’’ he says. ‘‘I think we could do a lot more [testing] and try and test in different places.’’
Under level 4, supermarkets were about the only places where people could be found in large enough numbers for efficient testing, but under level 3 there might be more opportunities.
But Roberts says it is encouraging that the community testing so far has not found anyone with Covid-19.
‘‘We have got very low numbers. We were locked down quite early and, of course, we can shut the border.’’
Michael Baker, professor of public health at the University of Otago, Wellington, says while it is good the community testing is showing negative results. ‘‘But the main thing to remember is that in theory we would have to do a vast amount of testing around the country to say there’s no circulating virus.
‘‘It’s much easier to prove something is here than not here. You literally have to do tens of thousands of tests to show convincingly nothing is present.’’
It does tell us it’s not floating around the community at high levels . . . That’s really good news.’’ Murray Cox, Massey University Professor in Computational Biology, on the lack of positive results from community testing