Herd immunity
With the rise of COVID-19, we’ve had to grapple with a whole new scientific vocabulary. Herd immunity (or community immunity) is often mentioned, but what does it mean? We’ll explain that in relation to the COVID-19 vaccine and other vaccines.
Herd immunity means enough of a population is immune to a specific infectious disease (a pathogen such as a virus or bacteria) that it stops the spread. If the virus fails to spread from person to person, fewer people will become unwell and die. And the pathogen will fall to very low levels.
Immunity can come from catching the disease and building antibodies: these have a memory that means the next time you meet the pathogen, your immune system recognizes it and rapidly kicks into action to defeat it, and you avoid serious illness or death. This is known as natural immunity.
The vaccine also gives immunity. Similarly, it teaches your immune system to recognize the pathogen and build memory cells so it’s primed to fight off any future invasions. This is desirable if the disease carries a high risk of serious illness or death, such as COVID-19. But natural immunity is preferred for something like the common cold, as it’s not a serious disease, so they haven’t developed a vaccine.
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Will the virus or bacteria eventually die out?
Tricky question. You would think it would – if everyone’s vaccinated, it can’t spread from person to person, so it just dies. But the answer is no – only smallpox has ever been entirely eradicated. Prevention of polio has had excellent success rates, with cases very rare due to constant vigilance from task forces worldwide, who isolate any cases and prevent local spread.
Pathogens can’t usually be eradicated because, firstly, not everyone is vaccinated for reasons including illness and personal choice.
Secondly, the pathogen itself wants to survive, it’s like a game of cat and mouse: it will keep mutating until it can find a way through your immune defenses. The lower the number of viruses or bacteria in the population, the less opportunity it has to mutate into a new version that is more deadly or avoids existing immunity.
Thirdly, immunity may reduce over time, either from the vaccine or from having had the disease – so you may need to keep up with boosters.
Fourthly, no vaccine offers a 100% protection guarantee – they just significantly reduce your chance of getting ill. Again, the lower the number of pathogens in the community, the less chance you will meet them.
Why are some people not vaccinated?
This may be because some people have allergies or ethical objections to a component of the vaccine components, or they have a lowered immune system. For personal reasons, they may choose not to receive it – you need to consent to any vaccine in the US – or they may have come from a country that doesn’t offer a vaccine.
Herd immunity aims to protect this unvaccinated group and the whole community, reducing the chance of mutant variants developing.
How much herd immunity is enough?
Herd immunity is designed to protect the very few who cannot get vaccinations if enough of the population is vaccinated. This varies with each disease. For measles, a highly infectious virus threatening serious long-term complications and death, 19 out of every 20 need to be vaccinated to protect that one unvaccinated person. That’s 95%. For polio, it’s 80%.
As COVID-19 is such a new illness, we don't know how much herd immunity is needed to keep unvaccinated people safe. While the vaccine rollout is underway, it’s thought that natural immunity (immunity from fighting off the disease) may wane over time, and there have been cases of people catching COVID-19 twice.
No formal decision has been made yet on vaccine boosters, but we will likely need to renew our vaccine regularly to uphold our immunity and protect against new mutant variants. This is what happens with the yearly flu vaccination program.
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