Herd immunity occurs when a high percentage of the population is vaccinated making it difficult for infectious diseases to spread, because there are not many people who can be infected.
For example, if someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone, and it will quickly disappear again. Herd immunity gives protection to vulnerable people such as newborn babies, elderly people and those who are too sick to be vaccinated.
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If you live in an area where vaccine coverage is low, and your child is not vaccinated, it’s quite likely that many of the people they come into contact with will not be vaccinated either. If one of these people gets an infectious disease like measles, they can easily pass it on to the other unvaccinated people around them, and in some cases the disease can then spread very quickly through the population. This is what happened during the 2013 measles outbreak in Wales.
However, our problem now is that there is no vaccination to protect against coronavirus – and there won’t be one for at least another 12-18 months. Therefore for the UK population to gain herd immunity, a large enough number of people – 60 per cent of the country, 40 million people, in the words of the chief scientific adviser Sir Patrick Vallance – will need to contract the virus and then recover. If that situation occurs then people will be less likely to get covid-19 in the future and the public can build up a resistance to it.
Elderly people
Elderly people are one of the key groups that rely on herd immunity to protect them, as they are more susceptible to disease. If they are able to isolate as much as possible during the coronavirus peak, once herd immunity is established isolation measures can be scaled back.
Given Sir Patrick has said he believes Covid-19 will become an “annual virus”, building up herd immunity will be vital. The only problem is, with no vaccination available, no one knows how long it will take for 60 per cent of the population to be infected, as no country has reached that figure yet. And as cases in China have slowed markedly to a handful, no country may ever get there given the extreme measures being put in place by those worst affected.
And as Oxford University’s Vaccine Knowledge Project points out, herd immunity does not protect against all vaccine-preventable diseases. The best example of this is tetanus, which is caught from bacteria in the environment, not from other people who have the disease.