JACK WATERFORD. Asymptomatic Covid cases will give us our next waves

May 12, 2020

As the Prime Minister and Premiers look to relax COVID-19 restrictions, we still need to be wary of the significant proportion of asymptomatic cases.

As the prime minister, premiers and chief ministers plan to make decisions about a staged relaxation of the Covid-19 rules, they are entitled to feel very satisfied with their efforts and their political courage. The proof is to be seen in the statistics of Australia compared with other countries, particularly in North America and Europe. We have had only a fraction of the mortality per million residents, and only a fraction of the incidence of mild and severe cases of the disease.

The credit goes to early political action to quarantine Australia, containment measures, and in securing the general cooperation of the population in measures to reduce the possibility of transmission of the Coronavirus. The nations which appear, so far, to have done best have been in South East Asia and Oceania, particularly Taiwan, South Korea, Hong Kong, New Zealand and Australia. Had Europe and North America been as successful in mobilising their resources against the virus, perhaps 200,000 more people would still be alive.

But it is far too early to celebrate, here or anywhere else in the world where the curve is going down, in our case to very low numbers of new cases, active cases, and deaths. There is, first, the job of cranking up the economy again, with a very close eye on the continuing risk from a virus which is still present in the population. Experiences with hotspots, whether with cruise ships, meatworks or nursing homes illustrate the potential of small numbers of cases to suddenly duplicate exponentially again, undoing all of the good work. Already some of the nations with a fairly good record — Singapore for example — are back fighting the smouldering remnants of a fire they thought they had put out. There, as here, the general population is as susceptible to the virus as ever — only a tiny fraction of the population has acquired any immunity — the strength of which we do not yet know — by having had the virus, perhaps asymptomatically, and then “recovered”. The hopes of the rest of us that we do not catch it, depend on quarantining, and isolation and containment programs that make exposure to the virus rare, and catching it less likely.

Doctors working on severe cases know by now a good deal about the way that the virus affects the susceptible — particularly those with compromised immune systems, or those who are old. The patterns of the disease among those who have been tested by screening programs — which, typically, are now finding a physical or serological case of the virus to every 99 cleanskins. From milder cases, which involved hospitalisation but not the need for transfer into intensive care units, doctors have found a wider array of symptoms than those usually described, including diarrhoea, digestive tract problems, and, in some cases, loss of a sense of smell and taste. A good many mild cases — about 40 per cent — do not have respiratory or flu-like symptoms, and, to date, have not been picked up by most screening programs, unless they have been pre-identified as suspects because of overseas travel, known exposure to a case or by case tracking.

In Canberra, for instance, words such as mass screening or random screening have been thrown around, but no member of the general public has been tested unless they have respiratory symptoms when they drive to locations such as EPIC for a test. The result is that none of the thousands of tests performed have much epidemiological significance: we do not know how many cases had the virus without symptoms, or had it with symptoms so mild that they did not seek medical attention (or, if they did, did not arouse the suspicions of their doctor).  Many of those who had flu-like symptoms of a non-respiratory nature, upset stomach, loss of appetite, vomiting , nausea and fatigue may have figured, on the basis of the information distributed by public health authorities, that they simply had a flu and would not, in the middle of a well-publicised public health crisis, trouble the doctors.

This might well have suited those managing medical services to those who were seriously ill. We had only a fraction of the cases for which the system — after seeing what happened in China and early in the European pandemic — had organised. Nonetheless the seriously ill were difficult cases and those concerned with them did not need the distraction of less serious cases, or cases that would resolve either with no treatment or only routine treatment. Indeed many of those managing the system rationalised their relative lack of interest in vigorous case finding in the general population by saying  that anyone with the virus, but no symptoms at all, or only mild symptoms, would be sent home for isolation. That is they would be subjected to much the same regime being asked of the general population — staying at home as much as possible, keeping their distance from others, and using good hygiene practices.

That may be alright so far as any immediate drag on hospital resources was concerned. But such people still have the power to transmit the virus to others, whether from person to person, or by shedding the virus on to some surface touched by another. The social containment measures may have had a pronounced effect in reducing such transmission, and the length of time the measures have been in force may mean that anyone who had the virus at the time such measures began is no longer spreading it. That is not necessarily true for a person to whom they passed it while they were contagious, or to a third or fourth generation recipient.

We know that the greater proportion of cases are asymptomatic — even if the carrier is spreading the virus to others. The evidence suggests that the asymptomatic are as virulent as any other. With the absence of good epidemiological studies anywhere, no one has much idea of the proportion of asymptomatic cases, but most assume that it is at least 10 and up to 100 times the number of serious cases. It may even be higher. It is also, probably, at least  10 times the number of mild cases requiring hospitalisation, or at least treatment that involves screening.

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