Scientists have known for years that measles can alter the immune system – but the latest evidence suggests it’s less of a mild tweaking, and more of a total reset, reports BBC.
Measles outbreak
It was late at night on 15 November 2019, on the Samoan island of Upolu – a tiny jade-green splodge in the Pacific Ocean. Government officials were rushing to attend a meeting in the sleepy harborside capital to discuss an urgent public health issue. By the end of the evening they had declared a state of emergency, with immediate effect.
Three months earlier, a member of the public had developed a characteristic red-brown blotchy rash after arriving on a flight from New Zealand, where there was an ongoing measles epidemic.
By 2 October, another seven measles cases had materialised. Schools continued as normal, with the small concession that prize-giving ceremonies were banned. Just over a month later, the outbreak had spiraled to alarming proportions – with 716 people infected, out of a total population of around 197,000.
Efforts to halt the spread
- Schools and businesses closed.
- Workers abandoned their offices.
- Residents were advised to stay in their homes.
- In a sinister echo of the red crosses marked on doors during medieval plague outbreaks, red flags popped up outside the homes of unvaccinated families across the country.
The epidemic was over – but the virus hadn’t necessarily taken its last victim.
Immune amnesia
Enter “immune amnesia”, a mysterious phenomenon that’s been with us for millennia, though it was only discovered in 2012.When you’re infected with measles, your immune system abruptly forgets every pathogen it’s ever encountered before.
The loss is near-total and permanent. Once the measles infection is over, current evidence suggests that your body has to re-learn what’s good and what’s bad almost from scratch.
About measles
Measles is an ancient respiratory virus, transmitted via aerosols and droplets, that’s thought to have first made the leap from cattle to humans around 2,500 years ago – possibly taking advantage of the crammed cities that were springing up across the globe.
For millennia, measles had free reign to plague the world’s children infecting nearly everyone before their 15th birthday.
When European colonists first made it across the Atlantic, the virus is thought to have been one of the new imports – along with others such as smallpox and typhoid – that wiped out 90% of the indigenous population of the Americas within a century.
Need for vaccination
Scientists have known for decades that even after they recover, children who have been infected with measles are significantly more likely to fall ill and die from other causes.
In fact, a study from 1995 found that vaccinating against the virus reduces the overall likelihood of death by between 30% and 86% in the years afterwards.
Measles is an infection of the immune system
Then in 2002, a group of Japanese scientists discovered that the receptor the measles virus binds to – a kind of molecular lock that allows it to enter the body – isn’t in the lungs, as you would expect for a respiratory virus. Instead, it’s on cells from the immune system.
A decade later, an international team of researchers – including Swart – decided to take a closer look. They tagged measles with a green fluorescent protein, infected macaque monkeys with it – and tracked where the green viral particles ended up.
“[We saw that] it infects many cells systemically,” says Swart.
“So, this virus causes a viremia, which means that then there’s virus in the blood – actually, white blood cells become infected and bring the virus to all the lymphoid tissues, which are your lymph nodes and your spleen, your thymus ,” he says, explaining that this confirmed that measles is an infection of the immune system.
Measles paradox
But this was not the end of the story. The team mostly found the receptor measles binds to a specific kind of immune cell, the memory T cell. Their job is to remain in the body for decades after an infection, quietly looking out for the specific pathogen each one was trained to target.
So, measles actively infects the only cells that can remember what the body has encountered before. What happens next is still baffling scientists to this day – so much so that it’s been called the “measles paradox”.
“Measles suppresses the immune system, and activates it at the same time,” says Swart. Though measles deletes immune memories, there is one exception to these losses. Oddly, the only virus you’ll definitely be able to recognise after falling sick with measles is measles itself.
Immune Response
Measles infections generate a powerful immune response against the virus, leading to lifelong immunity in the vast majority of people. And though no one yet knows why, this may be what causes immune amnesia in the first place.
Eventually, measles ends up replacing all your normal immune memory cells with ones that can identify it, and nothing else. This means you’re only immune to measles – while all other pathogens are forgotten.
A two-year gap
Since the discovery of immune amnesia, the pieces have started to fit together.
Once the immune system has lost its memory cells, it has to painstakingly re-learn everything it once knew. One population-level study from 2015 suggests that this process of recovery can take up to three years – which intriguingly, is around the time it takes infants to acquire immunity to everyday pathogens in the first place.
Children are at risk from a broad range of pathogens their bodies would once have been able to recognise. “Probably all those infections need to be experienced, again, to really repair all the damage there,” says Swart. “And every infection has another risk of disease development.”
A surprising impact
All this means that measles can have a profound impact on a population’s health, even years after an outbreak has disappeared.
Take Samoa. It’s thought that the 2019 outbreak of measles on the island stems from a traumatic and exceptionally rare incident years before, when two nurses mixed a batch of the measles, mumps and rubella (MMR) vaccine incorrectly, and two children died. This led to a widespread fear of vaccines, and as a result, just 30% of the country’s population had been fully immunised as of 2018.
And though the authorities did manage to get the measles epidemic under control, its impact may have lingered on.
Covid-19 was never given the opportunity to take off on the island
Modelling suggests that had it been able to, the population would have been at significantly higher risk as a result of the measles outbreak. According to these calculations, the islands’ legacy of immune amnesia could have increased the total number of cases by 8% and deaths by more than 2%.
Meanwhile, other modelling work has found that measles outbreaks occurring after Covid-19 vaccine rollouts could wipe out herd immunity to the coronavirus and lead to a resurgence of cases.
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Source: BBC
