Figure 1. The number of daily new confirmed cases of COVID-19 in several countries, between 20th January and 1st April 2020. For all countries, to varying degrees depending on the national rates of laboratory testing, the total number of cases would have been be substantially greater than the total number of confirmed cases. The large increase in confirmed cases in China during 11-15 February was the result of a change in reporting methodology and did not reflect changes in actual disease incidence. Note the levelling off and/or decline in the reported number of daily cases in China in February, and in some other nations in March, following the widespread introduction of physical distancing. (Reference 3)
However, if physical distancing is relaxed before 80-90% of the population have developed immunity (either as the result of natural infection or as the result of vaccination) there is a risk that the epidemic will recur. This risk may result from transmission of infection, either from unrecognised cases in New Zealand, or from people newly arriving in New Zealand with the infection. Because an effective vaccine is not expected to be available until late 2021, an option considered by many nations, including New Zealand, is to allow controlled spread of infection through the community. This would lead to a gradual increase in the proportion of the population who are immune, and potentially could result in 80-90% of the population becoming immune. Allowing spread of infection through the community would inevitably result in some patients developing severe disease, but careful ongoing regulation of physical distancing could ensure that the number of patients with severe disease did not exceed the human and physical resources of our healthcare system, particularly the capabilities of hospital intensive care units.
A possible strategy to allow continued spread of infection in the community, while consistently avoiding excessive numbers of severely unwell patients overloading the healthcare system, is illustrated in Figure 2. This strategy relies on periods of strong physical distancing interrupted by periods of less strong physical distancing. During the periods of less strong physical distancing the incidence of infection and disease would rise, while during the periods of strong physical distancing the incidence of infection and disease would fall. The duration of these alternating periods, and the strength of the physical distancing perhaps could be adjusted to ensure that the number of patients who require intensive care does not exceed the capability of the healthcare system to provide that care.