There has been a lot of talk in the media about levels and different strategies for the COVID-19 response in New Zealand. Below is a simplified look at what the terms mean.
Control of infectious diseases
Infectious diseases are diseases caused by infectious agents (pathogens) such as bacteria, viruses or parasites. Control of an infectious disease means reducing the number of people who become infected. Infectious diseases can be controlled using a whole variety of measures, such as immunisation, treatment of the infection or isolation of people who are infected or who may have been in contact with and infected person.
Health authorities can employ various strategies in the face of a pandemic to try and best protect public health. The choice of strategy is based upon a number of factors from its geographic location to resources.
What is elimination?
Elimination of an infectious disease means reducing the number of cases of the disease being spread within a geographic area or country to zero. Diseases such as measles have been eliminated in a number of countries including New Zealand. When a disease is eliminated in all countries it is called global elimination.
What is suppression?
Suppression is a control strategy that aims to keep the number of cases very low for as long as possible. This approach requires putting in place strong measures to reduce the opportunity for the infection to spread. This will include various forms of quarantine or isolation of infected people. There are many measures potentially available and it sometimes takes time before the effectiveness of these is known for a new pathogen.
Although there will continue to be cases of infection under a suppression strategy, the aim is that numbers will be low enough that the healthcare system will not be overwhelmed.
The success of a suppression approach relies on people who are infectious being quickly identified and on cases and all of their contacts being quickly isolated to prevent further spread. This can be challenging if cases aren’t identified, either because they are infected but don’t show symptoms (asymptomatic) or their symptoms are so mild that they don’t seek medical care. These people may unknowingly pass the infection to others and silent community transmission can occur. This is one of the most challenging aspects of an infection to manage. Infections which can be asymptomatic or where symptoms are very mild can be transmitted before anyone knows we have a problem.
Suppression delays the outbreak from happening, to allow enough time for vaccines or treatments to be developed. However, unless the infection has been eliminated globally or the population has been immunised with an effective vaccine, there is a risk that the outbreak will only be delayed until after the strong measures, such as border controls, are removed. These measures may therefore need to be in place for an extended period of time.
What is mitigation?
Mitigation aims to control an outbreak rather than eliminate the disease. The outbreak is controlled to ensure that the healthcare system isn’t overwhelmed by increasing the strength of measures as the outbreak progresses and number of cases increases.
For a mitigation strategy to be successful these increasingly strong measures must be put in place very quickly, at the right time. Strong measures such as border controls and lockdowns would be put in place if and when the number of cases increased to a significant level.
By allowing the outbreak to occur in a controlled way, the population builds up immunity to the infection – called herd immunity - prior to a vaccine becoming available. However, herd immunity may take time to build up and there may be very severe outcomes for some people who are infected.
Frequently asked questions
What about eradication? We have eradicated some diseases, right?
Disease eradication means permanently reducing the number of cases of a disease in the world to zero. This would be ideal, but only two diseases, smallpox in humans and rinderpest in cattle, have ever been eradicated. Eradication was only achieved after many years of sustained international immunisation programmes and thus relies on an effective vaccine being developed.
Why do we still say we have eliminated measles when there was an outbreak in New Zealand last year?
Even though we have had recent outbreaks of measles, New Zealand meets the World Health Organisation’s criteria for measles elimination. This is because all of the cases were linked back to travellers bringing the infection to New Zealand and we were able to control measles spreading in the community through measures such as immunisation and isolation of cases and contacts. We continue to have no cases of measles in New Zealand that are not linked back to returning travellers.