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 really 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 get 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 recognises 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. In a similar way, it teaches your immune system to recognise 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.
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 from the world. Polio has had excellent success rates, with cases kept very rare by constant vigilance from task forces across the world, who isolate any cases and prevent local spread. The reason pathogens can’t usually be eradicated is that, 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 defences. The lower the numbers of virus 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% guarantee to protect you – they just significantly reduce your chance of getting ill. Again, the lower numbers of the pathogen in the community, the less chance you will meet it.
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 take it up – you need to consent to any vaccine in the UK – or they may have come from a country that doesn’t offer a vaccine. Herd immunity aims to protect this unvaccinated group but also the whole community, as it reduces the chance of mutant variants developing.
Herd immunity is designed that if enough of the population are vaccinated, it protects the very few that are unable to get vaccinations. This varies with each disease. For measles, a highly infectious virus that threatens serious long term complications and death, 19 people 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 yet know how much herd immunity is needed to keep the 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 taken yet on vaccine boosters, but it’s likely we will need to renew our vaccine at various points to uphold our immunity and to protect against new mutant variants. This is what happens with the yearly flu vaccination programme.
While virus rates remain high, and not enough people are vaccinated, the UK policy is currently to protect those at the highest risk of severe disease and death by vaccinating them first. This is through their system of priority group, with the eldest and most clinically vulnerable offered the vaccine first, and the least vulnerable – fit and healthy young people – last in the queue. The lockdown is aimed at reducing the number of people catching the virus (and thereby reducing illness and death) while it gives a chance for more people to be vaccinated. Lower numbers of circulating virus also reduce the chance of a mutant variant emerging, which might escape the defence the virus offers. It’s a race against time to get everyone vaccinated and protected, and eventually declare community protection.