Herd immunity and Covid-19: what you need to know

What is herd immunity, how does getting vaccinated help and why is it so important?

As Australians are urged to get their Covid-19 vaccination to try and stop the spread of the virus, the words “herd immunity” are being bandied around.

What is herd immunity and how does it work?

University of Western Australia virologist Associate Professor Allison Imrie says herd immunity is about protecting as many people in the population as possible so there is less ability for a virus, such as Covid-19, to spread.

“We can achieve herd immunity by vaccination. If enough people in a community are vaccinated, there is less opportunity for a virus to move from person to person,” Dr Imrie says.

“If you don’t get enough people immunised, you don’t get herd immunity and the virus continues to spread.”

Why is herd immunity important?

Herd immunity doesn’t only protect the person who has been vaccinated  –  it protects people with a weak immune system, the elderly, children and the sick who are more vulnerable to the potentially serious impacts of a virus, such as Covid-19.

Royal Australian College of General Practitioners’ Dr Abhi Verma says once you have a certain proportion of people in a community that have been vaccinated, they can no longer spread it.

“There is a threshold where we can say that if a certain proportion of people are vaccinated, that will protect other people in their community to an extent,” Dr Verma says.

Melbourne’s Peter Doherty Institute for Infection and Immunity has advised the Australian Government herd immunity threshold would be reached when 80 per cent of eligible Aussies have been fully vaccinated against Covid-19.

But it’s not quite so simple, with some experts arguing children as young as five years old may need to get vaccinated in order for Australia to reach herd immunity.

Why is it so important to get vaccinated?

Dr Imrie says getting vaccinated isn’t a 100 per cent guarantee of safety.

“But if you are vaccinated and get the virus you are more likely to be asymptomatic or have a mild illness for a few days – you’re less likely to end up in hospital on a ventilator,” Dr Imrie says.

“You also reduce the amount of virus that you can transmit to people around you.

“Getting vaccinated and helping to create herd immunity is one of the most important things you can do to protect yourself, to protect the people in your family and to bring our lives back to some degree of normalcy.”

How herd immunity has worked in the past


Smallpox was a highly infectious disease that existed for at least 3000 years and killed millions of people. In 1967 the World Health Organization launched a global vaccination campaign to eradicate it and by 1979 the disease was declared eradicated.


Also called poliomyelitis, this viral infection can cause paralysis and death. The first polio vaccine was developed in the early 1950s. Immunisation is recommended for children as the disease remains a risk in some parts of the word. Australia has had no cases since 2000.


Measles is highly contagious and spread through coughing and sneezing. Serious complications can include pneumonia, encephalitis (brain inflammation) and sometimes permanent brain damage. In the mid-1970s, Australia began a vaccination program and the country was declared free of measles in 2014.

Zika virus

The virus is a mosquito-borne disease and there are concerns that infection in pregnant women can result in birth defects. There is no vaccine to protect against the virus and there have been outbreaks across South America. But by 2017, cases dropped dramatically because large numbers of people had been infected and developed natural immunity.

Written by Sarah Marinos.