Unvaccinated children aren't only at risk themselves—if too few people receive their vaccinations, the protection of "herd immunity" can be threatenedby Kelly Oakes / March 19, 2019 / Leave a comment
“No vaccine, no school” is how Italy’s health minister described a new law that came into force in the country last week.
In an effort to increase poor vaccine uptake, children under six will now be turned away from nursery if parents can’t prove they’ve had a range of mandatory vaccinations, including those for chickenpox, polio, measles, mumps, and rubella. While older children can’t be banned from school, parents can be fined up to €500 if they send their child in unvaccinated.
Measles vaccination rates in Italy reached as low as 80 per cent in some areas in 2017, when the law was introduced, and the number of children contracting the disease in April 2017 was ten times higher than it had been a year before.
Vaccination rates as low as 80 per cent are worrying—and not just for those children that go unvaccinated. The World Health Organisation says a rate of 95 per cent is needed in order to protect the population as a whole and stop outbreaks of diseases like measles.
That figure provides what’s known as “herd immunity”: the idea that, when enough people in a population are protected through a vaccine, that protection extends to others who have not been vaccinated. It works because vaccines don’t just provide you with individual protection against a disease, they also stop you spreading it.
If someone brings measles into a group of unvaccinated people it could spread to everyone, who would in turn pass it on to all the people they come into contact with, and so on. On the other hand, if all of the people someone carrying measles comes into contact with are vaccinated, it won’t get passed on and will find it much harder to spread.
Those who can’t be vaccinated because they’re either too young or don’t have a strong enough immune system rely on herd immunity to keep them safe. But it doesn’t take a huge dip in vaccination levels for the effect to drop off.
The vaccination rate required to keep a disease contained depends on how contagious it is. The key is to make sure each person passes it on to less than one other person, on average. Say someone with chickenpox spreads it to ten other people—to prevent an outbreak, a minimum of nine out of 10 people (90 per cent) would need to be vaccinated.
For measles that figure works out at 90-95 per cent, because each person tends to spread it to 10 to 20 others. For polio the figure is lower, at around 85 per cent, because it’s less contagious.
But these figures can be misleading. They are usually calculated by assuming that people in a population mix randomly, but that’s not how our lives work in reality. If 95 per cent of the whole UK population is vaccinated against chickenpox, but your social circle consists entirely of the 5 per cent who aren’t, you’re going to be in trouble. Some advocate 100 per cent vaccination targets for this reason.
There’s another pitfall, too: the idea that not everyone needs to be vaccinated might lead you to decide that you’ll rely on herd immunity instead of being vaccinated yourself. Of course, even putting aside altruistic notions of protecting vulnerable people, that’s not the best idea. If you’re unvaccinated and you come into direct contact with someone who has a contagious disease, you’ll be individually susceptible to it no matter the vaccine coverage among everyone else.
Overall vaccination levels of children in England fell for the fourth year running in 2017-18, though they are still higher than they were before 2011. Only 91 per cent of two-year-olds had been given the MMR vaccine, with rates in London the lowest at just 85 per cent—well below the 95 per cent figure recommended by the WHO.
Between January and October last year, there were 913 confirmed cases of measles in the UK, a steep rise from 259 in 2017. They were mostly in teenagers and young adults who’d missed out on vaccinations as children.
Parental choice isn’t the only reason some kids miss out on childhood vaccines: access to advice and health services also play a part. But it’s worth remembering that while getting vaccinated or not might seem like an individual choice, it’s one that has consequences for everyone else, too.