IN the 19th century, London used to be a haven for measles, with millions catching the disease. Karl Marx and his family were among them: in May 1854 they all got measles, the three Marx children aged between six and nine probably having picked it up at school.

Today, rates of measles in London are vanishingly low compared to the 1850s. However, last week on July 14 a report from the UK Health Security Agency (UKHSA) stated that London is at risk of a large measles outbreak in the near future.

Models predict that measles could spread in a London outbreak of 40-160,000 people, potentially causing dozens of deaths and the hospitalisation of thousands.

That risk in London is alarming, but it is small compared to the risk elsewhere in the world. Measles is not simply a disease of the past — it kills children every year.

In 2018 it killed 142,000 people worldwide, most of whom were very young children. Crisis situations such as war and famine are often followed by big outbreaks in refugee camps: Somalia and Somaliland in 2022 saw one of the world’s largest outbreaks. According to MSF, 95 per cent of deaths occur in low-income countries, particularly in Africa and Asia.

The key strategy against measles is vaccination. The first vaccines against measles were rolled out in the 1960s. They are cheap and effective, and since 2000 have averted an estimated 56 million deaths worldwide. They have allowed near-elimination of measles — but only in countries wealthy enough to maintain high vaccination levels.

Vaccination strategy in Britain is a good example. In the late 1990s the combined measles, mumps and rubella (MMR) vaccine was the focus of a lot of scepticism due to the discredited work of Dr Andrew Wakefield connecting it with autism, leading to a fall in vaccination levels which took around a decade to recover.

However, by 2016 and 2017 there were no reported measles cases in Britain, leading to World Health Organisation declaring that measles had been “eliminated.” But it wasn’t gone forever. Measles came back to Britain in 2018, then rose to over 1,000 cases in 2019. 

The Covid-19 pandemic led to a collapse in measles cases due to lockdowns and travel restrictions, with virtually none in Britain between May 2020 and January 2022.

However, it also led to a global fall in vaccination rates due to the disruption to normal healthcare and vaccination programmes. Today vaccination levels in children are at the lowest they have been in a decade. Most alarmingly of all, they are lowest in London, where the population density is high.

For an infectious disease, the reproduction number R represents the average number of cases each new case goes on to produce. Of all respiratory viruses measles has the highest possible R: in an unvaccinated society, R would be more than 10, and potentially even higher depending on how much people are in close contact with each other.

Reducing the number of unvaccinated people reduces R. Once enough people are vaccinated, a measles case is much less likely to have anywhere to go and R drops below 1. Though some people may still catch measles, the probability of a wider outbreak becomes near-zero.

Because measles has a high basic R, very high levels of vaccination are needed to make the probability of wider outbreaks very low. The World Health Organisation target rate is for 95 per cent immunity by the age of five, meaning that 95 per cent of children should have had two doses of the MMR vaccine. In England the current level in official data is 85.5 per cent; in London, it is only 74.1 per cent.

That 10 per cent difference can mean a much more than 10 per cent difference in the risk of an outbreak. Because in London as many as a quarter of five-year-olds may not be fully protected against measles, that means any spark of a new case is much more likely to set alight a wider outbreak.

To understand why the UKHSA’s warning of an outbreak in London has such a large range, it’s important to understand that no data is perfect. The official vaccination rates are effectively the worst-case scenario for an outbreak when taken at face value.

Actually, it is certain that some children will in fact be vaccinated despite not having been officially reported — the data comes from individual GP practices reporting vaccination rates, which can have errors. How many of them there are is unknown.

That number has a big effect on how likely an outbreak is. If 50 per cent of unvaccinated five-year-old Londoners are actually vaccinated then R would be 0.9; if it’s 10 per cent, then R is currently about 1.6. Though we don’t know what the real percentage is, the most likely scenario seems to be that R has exceeded 1 in London, even if we can’t narrow down a value with high precision.

R above 1 doesn’t mean an outbreak is inevitable, but that without further action one is likely. So far this year, up to April there were 49 laboratory-confirmed measles cases, of which two-thirds were in London. None has produced a wider outbreak in London yet.

However, with the current vaccination rates below the crucial 95 per cent, the modelling reported by UKHSA suggests that a large outbreak is possible. If more cases continue to pop up in London, and vaccination coverage isn’t increased, then modelling predicts it is only a matter of time. 

Vaccine sceptics are dubious of the benefits of vaccination, but historical analysis backs up the claim that measles immunity levels predict outbreaks.

One 2019 study by researchers at the London School of Hygiene and Tropical Medicine used estimated immunity levels from blood samples combined with contact mixing patterns (data about how people interact with others) in 17 countries.

Taking into account the amount of mixing between people, the general vaccination level was a good predictor of whether a country would experience outbreaks in the following decade.

Of course, it wasn’t perfect: there are no certainties here, just probabilities. Indeed, some countries were noticeable outliers. At one extreme, Latvia had low immunity of 71 per cent but few reported cases; Spain and Israel had levels above 95 per cent but thousands of cases.

That might be due to imperfect data but there might be a more sobering explanation: inequality. Society is not a single undifferentiated mass. Different communities may have different social networks and different levels of vaccination, putting them at dramatically different risks.

As UKHSA report, vaccine uptake varies by “ethnicity, deprivation and geography.” Scientists don’t usually mention class, but it is another crucial factor.

Even an average vaccination rate of 99 per cent can be consistent with some people — such as particular migrant groups, travellers, or orthodox Jewish communities — having a much lower rate, and producing small but still devastating outbreaks. An indiscriminate infection turns into a discriminatory killer, set loose by society’s inequality.

The key is access to vaccines for all. The most dangerous “vaccine scepticism” is not from those who reject vaccines for themselves, but from those who are dubious about expanding global programmes.

The ongoing tragedy of thousands of children dying every year is an urgent crisis, even more serious than the (real) risk of outbreaks in London. In the 21st century, those on the left should continue to push for intensified programmes to ensure fair access to vaccination — both in this country and around the world.