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Patchy COVID-19 testing may underestimate cases in Australia

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Chief Medical Officer of Australia, Brendan Murphy (image via YouTube).

We must rigorously map the scale of the coronavirus to properly determine its prevalence in the community, writes Dr Kim Sawyer.

AUSTRALIA TENDS TO copy other countries. We are not a cargo cult of others, but often exhibit some of the elements. In a pandemic, we follow the best practice of other countries. But we must fine-tune.

We have different qualities. The environment, the spatial population distribution and being in the Southern Hemisphere render us different. The last factor may become the most important. We were protected early because of the distance from the major epicentres of infection.       

Now, as we open up society, we enter winter, the most likely time for the spread of a virus. COVID-19 is not the flu, but it behaves like the flu. We have may have got the timing wrong. We may have locked down too early.

To be sure, we had to lock down early to limit the contagion. But we needed to understand we were not Wuhan in China, Italy, or the U.S.

Most cases have been imported. We have had only a few clusters. Yet we should not fool ourselves. We have not mapped the virus. To map the virus, there needs to be extensive antibody testing.

Countries that have conducted extensive antibody testing have shown just how prevalent the virus is. Samples of the population in New York State show 14.9% testing positive to coronavirus antibodies and 24.7% in New York City. Samples in London have found 17% testing positive and 5% in the rest of the UK.

The story is the same wherever extensive antibody testing has been conducted. The percentage testing positive in Stockholm is 7.3%, in Russia (principally Moscow) the percentage is 14% and in Paris 10%. We are not those cities and countries. 

Almost certainly, however, we have underestimated the rate of infection. Victoria with 1,618 confirmed cases has an infection rate of 0.025%. Expressed another way, Boston has an infection rate about 400 times that of Melbourne.

Do we really think we have been that lucky? We simply do not know.

The problem is with our testing strategy. There are two problems. First, we are testing those who select themselves to be tested rather than selecting them.

When we want to know how people will vote, we do not ask them to volunteer. We sample them. Sampling randomly for the virus across the population samples the symptomatic and asymptomatic. 

The second problem is the test being used. There are two general types of test. The Polymerase Chain Reaction (PCR) tests that detect whether an individual has the virus and antibody tests that detect whether an individual has been exposed. Antibody tests are not designed to detect an active infection, rather the legacy of an infection. The immune system takes time to respond, as much as 15 days.

Antibody tests map the virus, where it has been and where it is likely to go. They are exploratory tests. We are not using them enough.

What is the problem with PCR tests? The problem is that of false negatives. If a person has cancer and tests negative for that cancer, it affects only them. If a person is infected with COVID-19 and tests negative, it affects not only them but everyone in contact with them. PCR tests have consistently been shown to produce a high proportion of false negatives.

In one study in the American College of Cardiology Journal, it was shown that the false-negative rate was 67% within the first five days after exposure, lowest eight days after exposure (21%) and then 66% 21 days after exposure.

What this means is that 20 days after exposure, the PCR tests only detect a real infection one in three times.

Australia has conducted more than 1.3 million tests. But they are all PCR tests. Do we not understand that some of the negative test results may be false?

An early case in China was illustrative. An asymptomatic carrier infected five relatives, two of whom developed severe pneumonia. She tested negative three times in four tests. The test used was a PCR test.

There seems to be a bias against antibody testing in Australia. Stories have been run in various media against the tests. An article in Sydney Morning Herald suggested antibody tests were no more reliable than a flip of the coin, yet the article acknowledged that the tests detected antibodies correctly 84.8% of the time when people were tested 20 days after the onset of symptoms; that is, only 15% were false negatives.

Many reliable antibody tests can be used.            

Why is Australia not using them?

The problem of testing has been compounded by price gouging. At the peak of the pandemic in Australia, it is alleged that private pathology companies threatened to refuse COVID-19 testing, forcing the Federal Government to increase by four-fold its payment for each test. Some pathologists may have benefited from the pandemic.

We now face a real dilemma. We have not adequately mapped the virus in Australia. We have not tested as other countries have. We have not done random sampling across the population to determine the rate of infection and who has been exposed. And it is unlikely the virus has been eliminated. It is also unlikely only 0.025% of our population has been infected.

The dilemma we face is whether the asymptomatic who have been infected and not detected are still infectious.

Dr Kim Sawyer is a senior fellow in the School of Historical and Philosophical Studies at the University of Melbourne.

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