Statistics from Asia suggest the weakness of the coronavirus Covid-19 is that it can be controlled by rapid case contact management.

OPINION: Most infectious diseases have an Achilles heel – the secret is to find it. The question is, if we don't have a drug or a vaccine for Covid-19, is there something else we can do to beat it?

Some people estimate that, without any intervention, 50 per cent of the Chinese population might have developed the virus, and if we use a 1 per cent case fatality ratio, then the expected number of deaths would have been 7 million. Instead, there have been fewer than 3500 deaths reported across 1.4 billion people.

This occurred in the absence of any effective drug or any vaccine. Therefore, something they did in China must have exposed an Achilles heel of the virus.

As researchers, we are trained to observe and carefully interpret multiple lines of evidence when analysing a health problem. In the current situation, evidence is accumulating rapidly.

SUPPLIED Covid-19 death rates by age group.

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We can use health statistics describing what happened within China, comparing Hubei with the rest of the country. We can then compare these experiences with what happened in other Asian countries that have been successful in containing the virus. We can also draw conclusions from what is happening in Europe and the United States. Finally, we can attempt to reconcile this information with our evolving understanding of the properties of the virus and how they are different to the Sars coronavirus and to influenza.

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SUPPLIED Professor Philip Hill, of Otago Medical School, says New Zealand should urgently expand its case contact management workforce.

There are three key conclusions we can draw from the health statistics. Firstly, we can conclude from Hubei and Italy that not acting quickly, and limiting activities to simply diagnosing and managing cases, is potentially disastrous.

Secondly, we can conclude from South Korea that even quite large outbreaks with community transmission can be brought under control.

Thirdly, when we look at the various different measures employed by Asian countries who have been successful, there is one common intervention that stands out – it is called rapid case contact management. This is simply rapid early diagnosis of as many cases as possible, and rapid early isolation of all their contacts. It doesn't even need to be 100 per cent perfect.

Supplied Associate Professor James Ussher, of Otago University.

There are two key properties of the virus in relation to other viruses that are relevant to this discussion. Firstly, evidence is accumulating that this coronavirus has an important difference to Sars – it has peak transmissibility very early after the development of symptoms, whereas Sars didn't reach peak transmissibility for several days after symptoms developed.

This means that a strategy limited to just diagnosing cases leaves it too late to stop ongoing transmission of the Covid-19 virus. Secondly, it is becoming clear that Covid-19 is not as efficient a pathogen as influenza in terms of time to causing disease – it takes on average five or six days for it to start causing symptoms.

This means that there is a bigger window of opportunity to find all the contacts of the case and isolate them. These two key properties could explain why rapid case contact management is such a powerful tool to combat Covid-19.

Its Achilles heel appears to be an exquisite sensitivity to rapid case contact management, even after it has established large outbreaks. This ties in with what we are learning about the properties of the virus.

Influenza pandemic plans use rapid case contact management early, and move to case diagnosis and management only (abandoning contact tracing and isolation) soon after community transmission of the virus has been established. Evidence from Asia, and the particular properties of Covid-19 suggest that this would be disastrous for New Zealand.

We have some of the best public health practitioners in the world, who do case contact management of infectious diseases all the time. What is required, apart from increasing capacity and sensitivity to diagnose Covid-19, is an urgent large expansion of our case contact management workforce.

There are various ways to expand this workforce – including, for example, very fast (one to two days) training of people for this specific task under supervision, including possibly our own nursing and medical students. We have the opportunity to activate such a plan now before our current workforce becomes overwhelmed.

* Philip Hill is McAuley Professor of International Health at Otago Medical School. James Ussher is an associate professor and immunologist at Otago University.