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Q: Can you explain the concept of herd immunity?

A: Herd immunity occurs when the number of people immune to infection in a population is sufficiently large enough to prevent infection of the non-immune people. Let’s say we have a disease an individual has introduced into an entirely susceptible population and they infect two new people during their entire infectious period. Assuming recovery from infection leads to immunity, then the number of immune people in the population steadily grows. As the epidemic progresses, more of an individual’s contacts will be immune, making it harder to pass the infection on to new people. When an infected individual is only able to, on average, infect less than one individual, the epidemic will die out. This occurs because as an infectious individual recovers, since they infected less than one other person, there will be fewer infectious individuals remaining. So, herd immunity (often generated by vaccination, rather than infection) can lead to a disease dying out.

Q: Is there any evidence to support the theory of young people (who are at less risk of death or serious illness) getting COVID-19 to help control the disease?

A: It’s a fairly dangerous concept to think that we should allow people to become infected when we know so little about this virus. While the evidence seems to be generally supporting limited COVID-19 disease in young people, we know there are instances of young people becoming infected and getting severe illness. There is also much uncertainty around the role that young people are playing in transmission. Young people generally have a higher number of contacts than other age groups - even if they were less infectious than other age groups due to fewer symptoms (but infectious to some extent), they could still spread infection. In addition, there is too little information at this stage to know whether COVID-19 produces immunity and how long that immunity might last. Herd immunity is more difficult to achieve when immunity is short-lived.  

Q: Is Australia heading for the same situation as Italy and Spain? What does the modelling tell us?

A: Australia introduced control measures early in the epidemic, such as banning mass gatherings, requiring returned travellers to self-isolate and providing information for individuals on how to best protect themselves from infection. These actions have slowed the spread of infection in the Australian population and are helping us manage the effect of COVID-19 on our healthcare system. At this stage, it’s looking promising that we’ll be able to avoid our healthcare system being overwhelmed as we have unfortunately seen in a number of other countries, but we can’t become complacent.

Modelling provides a useful framework to consider the impact of a range of interventions and, particularly in the early stages of an epidemic when information is limited, allows the uncertainties associated with each approach to be quantified. Modelling is helping the Australian Government and the states and territories prepare for the likely demand for different healthcare channels, under a number of possible interventions. As the epidemic progresses, and more information is known about COVID-19, estimates can be refined. Read more about Doherty Institute’s modelling.

Q: What is flattening the curve and why is it important? 

A: Flattening the curve means slowing the spread of disease down so that the demand on healthcare channels is more evenly distributed over time, rather than being experienced as a sharp peak. If we can reduce the number of infections occurring each day, then more of the people who need hospital beds and intensive care beds will be able to access them.

Q: What are the best ways for individuals to do this? What are the best ways for communities to do this?

A: To flatten the curve, individuals need to reduce their risk of infection. One way to do this is to practise good hygiene, including washing hands thoroughly and frequently, not touching faces, and coughing and sneezing into elbows or tissues. Another way of reducing infection is through physical distancing. Each state and territory has their own restrictions on being out of isolation, for example, there are only four reasons why Victorians should be leaving their homes: food and supplies, medical care and care giving, exercise and work or education.

Q: Should restrictions be relaxed?

A: It would be taking quite a risk. We have crushed transmission in our population, so now we have a population that is almost entirely susceptible to the illness. If we relax the restrictions, we could see a huge increase in transmission. Understanding the next steps to be taken will require careful planning.

Stay up-to-date on the latest COVID-19 advice.

The information above is correct as of 15 April, 2020.