Will we see many more cases of Covid-19? Can we eliminate the disease from New Zealand? The world?
So far, New Zealand has done exceptionally well in pushing the transmission rate well below the magic value of 1 new infection from each infected person, whereas some countries have at worst been approaching 5, which means the disease spreads like wildfire.
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As New Zealand has moved to alert level 3, the number of cases reported per day is close to zero, but the amount of contact among people is going to increase. It would be remarkable if we saw no more clusters of infection arise. What is most likely to happen is that we will get a blip of new cases starting in the second week of level 3, from May 5 onwards.
This is because although the testing programme has been very extensive relative to the size of the outbreak here, it cannot detect every case. With an incubation period of 5 to 10 days, contacts of the undetected infected people will start to show up with the disease from May 5.
Finding new cases would actually be helpful in moving forward, by flushing out any hidden clusters of infection. With a "zero tolerance" policy in which prompt action is taken to isolate infected people and rapidly trace contacts, it should be possible to get the true level of infection down to zero within perhaps three months, with a very low risk of the resurgence that has occurred in other countries when they have lifted the lid a little. It will take a while after that to provide confidence that the infection has been eliminated, and we may still have an occasional case for some time, perhaps arising from an infected traveller who is still excreting virus after completing quarantine. The virus seems to remain active longer than usual in a few people.
Australia has had a softer lockdown policy than New Zealand, and has therefore set up a situation where there may well be more hidden clusters of infection in the community, since they will not have pushed the transmission rate as low as New Zealand. Unless Australia has been extremely lucky, they are likely to have cases continuing on for several months longer than New Zealand, and may have greater difficulty reaching elimination.
New Zealand is likely to gain economic benefits from rapid elimination of the virus and hence speedier return to normal business activities.
Each time a new disease enters the human population, we have to watch carefully for any "sting in the tail" of infection, where the virus has effects on people which take time to detect. Loss of smell and taste in infected people has already been reported widely in this pandemic. There is now growing evidence that the coronavirus also affects blood clotting in the body. This seems to influence the difference in severity of disease between people, and is now also being identified as the likely cause of serious strokes in unusually young people who have become infected but have not necessarily had severe pneumonia.
The virus affects multiple organs other than the lungs, so there may be further stings yet to show up, and New Zealanders have therefore gained from keeping the number of cases
very low. If New Zealand eliminates the virus it will be the first country to achieve freedom.
Can we eliminate the virus from the entire human population, as happened with the Sars coronavirus? That virus is still in bats in China, but no longer in people anywhere in the world. China, despite having the largest population of any country, has succeeded through intensive lockdown in getting surprisingly quickly to the point where almost all of its 10 to 20 or so reported cases per day of Covid-19 are in arriving travellers. So elimination of the disease in large populations is possible.
The problem lies in countries where implementation of control is haphazard, which provides gaps for the virus to maintain itself. "Herd immunity" by natural spread will never eliminate infection with this particular virus.
A highly protective vaccine would almost certainly make global control possible. However it is far from certain that such a vaccine can be developed, and if it can, it will take a long while to be distributed widely enough to stop the disease globally.
Meanwhile, New Zealand will provide the world with a very valuable lesson in the effects of a carefully implemented control and elimination policy. If we succeed, so can other countries.
• Professor Roger Morris is an epidemiologist who has been involved in around 30 disease control and elimination programmes throughout the world, involving both animals and people, and training doctors and veterinarians in many countries to deal with emerging diseases. He is currently advising the World Bank on part of its planned investment of up to US$160 billion to assist countries to recover from Covid-19, and to prevent a future pandemic.