What is herd immunity and will it help protect us against coronavirus?

With the coronavirus pandemic dominating global headlines, a number of new terms have entered the popular lexicon – among them is the notion of “herd immunity”.

But what is herd immunity and will it help us get through the current crisis?

What is herd immunity?

Herd immunity, sometimes referred to as community immunity, is a situation in which a sufficient proportion of a population is immune to an infectious disease to make its spread from person to person unlikely.

“It’s generally understood in terms of vaccinations,” says Timothy Newscome, Associate Professor and virologist at Sydney University.

“You need to achieve a certain level of protection in a large population, and that will then block the transmission between susceptible people. They’re effectively protected by being surrounded by people who cannot transmit an infection,” he told Yahoo News Australia.

“The more contagious a disease is, the higher the level of people who are protected needs to be in order to achieve herd immunity.”

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When it comes to the novel coronavirus, in a best case scenario a large proportion of the population, predominantly younger and healthier people, catch the virus, recover, and build up their immunity in order to help protect more vulnerable citizens from contracting the disease.

Some health officials in the UK have suggested that about 60 per cent of the population would need to build immunity through contraction of COVID-19 for herd immunity to be effective.

Passengers arrives at Sydney airport on March 16. Source: Getty
Passengers arrives at Sydney airport on March 16. Source: Getty

Is the UK trying to rely on herd immunity?

Last week, the UK appeared to be forging its own path with a strategy to fight COVID-19 that was far more reliant on herd immunity than other countries.

The term soon began trending on social media after the British government's chief scientific adviser, Patrick Vallance, went on BBC radio and told the country one of “the key things we need to do” is to “build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission.”

The idea that the UK was embracing herd immunity caused widespread concern among the public and scientific community with epidemiologist William Hanage, from the Harvard's Chan School of Public Health, saying he thought it was a joke when he heard about the apparent plan.

“Your house is on fire, and the people whom you have trusted with your care are not trying to put it out,” he wrote in The Guardian. “The UK government has inexplicably chosen to encourage the flames.”

More than 200 scientists also wrote an open letter to the government urging it to reconsider the “risky” plan, saying it would put unbearable pressure on the country’s health system.

“Going for herd immunity at this point does not seem a viable option, as this will put NHS at an even stronger level of stress, risking many more lives than necessary,” the letter said.

British Prime Minister Boris Johnson and the UK government have since dramatically changed their tune.

On Tuesday (AEST) Mr Johnson called on the country to abide by strict social distancing measures and implored people to work from home. Meanwhile at-risk groups such as the elderly will be asked within days to stay home for 12 weeks, the BBC reported.

The UK, it seems, has completely backed away from what was earlier being described as a major gamble on herd immunity after the government fine-tuned its modelling on the demand for intensive care units in hospitals.

About the same time as the UK changed course, the Imperial College of London released a detailed paper which concluded that “epidemic suppression is the only viable strategy at the current time”.

More than 1,500 people have tested positive for COVID-19 in Britain so far.

Can herd immunity save us from the worst case scenario?

There has been some early progress made on vaccines but with human trials needing to be successful before a vaccine can be manufactured for common consumption, any potential herd immunity effect in the near term will only come via widespread infection.

“Herd immunity can also be achieved by people actually getting infected by the actual disease, if that elicits good protection,” Prof Newscome told Yahoo News.

“What is not clear at the moment is how good that protection is from the novel coronavirus.”

On March 17, this is how the known coronavirus figures in Australia stand. Source: Yahoo News Australia
On March 17, this is how the known coronavirus figures in Australia stand. Source: Yahoo News Australia

Scientists don’t know yet if people build an immunity to the novel coronavirus after being infected just once, but early signs do look promising, according to Prof Newscome.

“The evidence at the moment suggests there is no credible evidence for re-infection,” he said.

There were early reports of people ridding themselves of the disease only to test positive at a later date. However Prof Newscome said it appeared likely those were false negatives and the person had not actually recovered from COVID-19 in the first place, and thus was not re-infected.

“They seemed to be the result of a false negative diagnosis,” he said. “A patient got a positive result, then a negative result, then a positive result, but actually that middle result was a false negative.

“That’s not to say it can’t happen. But my understanding at the moment is there is no credible cases of re-infection.”

If the chances of re-infection are in fact small or nil, that bodes well for the utility of herd immunity in protecting against the long term spread of the virus.

As children don’t exhibit the worst of the coronavirus symptoms, some have speculated the federal government’s interest in keeping schools open is, in part, aimed at boosting herd immunity.

“Keeping the schools open is a defensible option,” epidemiologist and former deputy chief medical officer John Mathews told The Australian Financial Review.

“One possibility is that because childhood infections are so mild, they will transmit smaller doses of virus, and this would induce milder illness in their parents and grandparents at home.”

Can herd immunity stop a second wave of infection?

When dealing with this outbreak, medical experts appear unanimous in the need to ensure it doesn’t overwhelm the healthcare system, as Australia, like other countries, is worried about a shortage of testing kits, ventilators and hospital beds.

“Our priority has to be making sure our medical infrastructure can handle the number of severe cases we’re going to get, be it the first wave or the second wave,” Prof Newscome said.

Initially, the UK built its strategy around a concern that the virus would return in a second wave of infection, suspecting it might even happen the following winter.

While it’s thought this coronavirus spreads more rapidly in the colder winter months, more research is still needed on this.

“Given the seasonality of this virus is not well understood, gambling on the future is a risk,” Prof Newscome said of such a policy.

A group of elderly shoppers wait outside a Woolworths to get essential supplies in Sunbury on March 17. Source: Getty
A group of elderly shoppers wait outside a Woolworths to get essential supplies in Sunbury on March 17. Source: Getty

For a vast majority of medical experts, herd immunity is a possible and eventual by-product of the outbreak (and ideally a vaccine), but not something to be engineered in the meantime. If so, it would likely cost the lives of thousands of vulnerable people.

“Whatever strategy we employ will ultimately be successful due to the acquisition of herd immunity,” Prof Newscome said. But the health infrastructure of the country and the health of those vulnerable people is the paramount concern.

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