Why is herd immunity so important?
Australia’s best chance of bringing the coronavirus pandemic chaos to an end could be derailed by a possible stumbling block.
One of the benefits of vaccination is that it can help protect the entire community – even those who don’t get vaccinated – if “herd immunity” is reached.
Herd immunity is achieved if enough of the population is vaccinated or immune to the disease, as this slows the spread of the virus because it can’t easily find more people to infect.
Those who can’t get vaccinated due to health or other issues also benefit because they won’t get the disease if people around them aren’t infected.
Vaccinating against COVID-19 is the easiest way for Australians to get their normal lives back, but millions are hesitant to get the jab.
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RMIT University professor of immunology Magdalena Plebanski said achieving herd immunity would allow people to return to work and live a more normal life.
It’s particularly beneficial for those people who can’t get vaccinated due to a poorly functioning immune system or other health issues.
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For example, people who have severe allergies have been advised not to get the Pfizer vaccine after bad reactions in two people. It’s also not clear whether those who are pregnant can get the vaccine.
“If you think of vulnerable people you might be thinking of the elderly, but there are other groups who will benefit, such as those with diabetes, cardiovascular disease and other pre-existing conditions,” Prof Plebanski said.
The number of people who need to be vaccinated depends on how effective the vaccine is, how infectious the virus is, and can also vary depending on the dynamics of a population.
The more infectious the virus is, the more people are required to be vaccinated to achieve herd immunity.
The behaviour of people can also make a difference. In Australia, where people generally don’t live in crowded accommodation, the effectiveness of the vaccine may not have to be as high as for places where it’s more difficult to socially distance.
But concerns have been raised about whether Australia can achieve herd immunity using only the AstraZeneca vaccine.
HOW DO WE ACHIEVE HERD IMMUNITY?
The vaccines that are available from Moderna, Pfizer and AstraZeneca are new products and so it’s difficult to judge exactly how effective they will be.
There are also other factors to consider.
Based on current estimates of how infectious COVID-19 is, a vaccine with efficacy of 90 per cent, would require about 70 per cent of the population to be vaccinated, to achieve herd immunity, infectious diseases expert Professor Raina MacIntyre told news.com.au.
“If a mutant strain that is more infectious takes off, those percentages will be higher,” she said.
Prof MacIntyre is head of the biosecurity program at the Kirby Institute at the University of NSW, and is concerned that the AstraZeneca vaccine, which will likely be rolled out early this year, is not strong enough to achieve herd immunity in Australia.
Phase III clinical trial results published in The Lancet last month, showed the AstraZeneca vaccine has an efficacy rate of 62 per cent. This compares with other vaccines from Moderna and Pfizer, which are about 95 per cent effective.
Prof MacIntyre noted the results were calculated by looking only at those who got symptoms but many people don’t get symptoms when infected with COVID and she believes it’s necessary to look at the efficacy against all infection when assessing herd immunity.
“From the available data, the AstraZeneca vaccine efficacy against all infection is somewhere between 38-50 per cent (depending on which data you look at, given different study sites measured infection in different ways),” she said.
“If this is the case, the efficacy is unlikely to be high enough for herd immunity, even if 100 per cent of people are vaccinated.”
The World Health Organisation has set a preferred efficacy rate of 70 per cent for any vaccine and a minimum of 50 per cent. However, these targets do not what reflect what it will take to reach herd immunity.
IT WILL TAKE A WHILE
Professor Allen Cheng, who leads the government’s Australian Technical Advisory Group on Immunisation, told news.com.au’s Samantha Maiden in January that it may also take a bit of time for Australia to vaccinate enough people.
At the moment there is no vaccine available for children and they represent about 20 per cent of Australia’s population.
There will also be between 10 to 20 per cent of the population who can’t get the vaccination for health or other reasons, or don’t want to get vaccinated.
Pregnant women for example are being advised not to get vaccinated as there is not enough data about whether it’s safe for them to do so.
Those with a history of severe allergic reactions are also being advised not to get the Pfizer vaccine.
A COMPLICATING FACTOR
One thing that could derail plans for herd immunity is if it’s proven the vaccines don’t stop transmission of the virus.
The vaccine trials only tracked how many people got sick with COVID-19 and so scientists can only prove vaccines stop people from developing symptoms.
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The vaccines work by teaching the body how to recognise and fight off the disease but it’s unclear how quickly this response works and whether it will stop the virus multiplying in the nose (where it first enters) and being sneezed out.
It’s possible some vaccinated people could be infected with coronavirus but not have symptoms and then pass the virus on to others. If this is proven, then current vaccines will not provide herd immunity.
This is one reason people are being advised to continue social distancing and wearing masks even if they have been vaccinated.
It’s also unclear how long people keep their immunity to the virus and so people may eventually need to be vaccinated again.
Prof Cheng said what happens in Israel over the next month or so will give experts an idea of how effective vaccines are at preventing transmission of the virus.
Israel is only using the Pfizer vaccine but they have rolled out lot of doses and the country also has a lot of coronavirus cases.
“I think the benefits of that vaccination program in Israel will become clearer, probably in the next couple of months,” Prof Cheng said.
Information about the AstraZeneca vaccine will probably be available from countries like England by the middle of the year, he said.
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HERD IMMUNITY IS NOT THE ONLY GOAL
Developing herd immunity is not the only reason to get vaccinated and getting the jab will still save lives.
A vaccine with a 50 per cent efficacy rate should still stop about 50 per cent of people from falling sick, and many more from getting severely ill.
For example, during the Phase III trials of the AstraZeneca vaccine, no one who had the vaccine needed to be hospitalised so it was very effective at preventing severe disease.
“The benefit of getting vaccinated is so you don’t get sick if there is a COVID outbreak in Australia again,” Prof Cheng said.
“In terms of when we might be able to go back to a more normal life, that really remains to be seen.”
Prof Plebanski said vaccines could also slow down the spread of the virus even if they don’t stop everyone from transmitting it.
She said people were more likely to spread the disease if they were really sick and “shedding” a lot of virus particles while coughing and sneezing. If people were less sick and decreased their viral load, this may make them less infectious.
“By everyone taking the vaccine you are saving lives,” Prof Plebanski said. “It’s a responsible thing to do in a society where there’s a deadly virus around.”
The AstraZeneca vaccine is set to be the dominant jab in Australia because it can be manufactured in the country and the government has secured enough doses to vaccinate the entire population.
Australia’s chief health officer Paul Kelly told reporters on January 13 that while it may not be as good as other vaccines, it was effective enough.
“The choice is not whether one is better than the other, it’s which one is available to give the maximum rollout of vaccine … to protect lives this year,” he said.
Prof Kelly said not many people in Australia had contracted COVID-19 and this put the community at greater risk.
“We’ve had so few cases in Australia over the last year compared with many other countries that we are essentially completely non-immune,” he said.
This lack of immunity makes it even more important for Australia to roll out as much vaccine as possible, he said.
charis.chang@news.com.au | @charischang2
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