Measles virus or ‘human morbillivirus’. I was considered just another pox, like chickenpox or smallpox, until Rhazes of Persia provided the first scientific description of me around 900 A.D. Since then I have been a scourge of humanity.
I’m the causative agent of measles (or rubeola) in humans, a systemic infection that leads to high fever; immune cell dysfunction and immunosuppression and potential nervous system problems. Not to be confused with German measles; that’s rubella virus. Before vaccination I used to kill over 2 million people a year but now I only kill fewer than 200,000.
I am an enveloped virus (see right) with a long single-stranded negative sense RNA genome, which encodes less than ten genes. In particular, I look like other members of the taxonomic order: Paramyxoviridae. My cousins include canine distemper virus, rinderpest virus, mumps virus and even the deadly, re-emerging Nipah virus.
My genome is encased in a protein shell or capsid, which is surrounded by a lipid bilayer, otherwise called an envelope. Proteins are studded into this envelope, which I use to get inside and infect your cells.
You sound terrifying. Why have you been in the news lately?
In 2014, the USA reported over 600 cases of measles, which is about six times the average number of cases over the last decade, and was mainly associated with the California outbreak. Other outbreaks, unrelated to this one, have led to the USA experiencing multi-state measles outbreaks, which it is still dealing with. The vast majority of cases were in people who were not vaccinated. In 2015 there has been nearly as many cases in the first two months than in the majority of years in the last ten years
But remember that measles goes largely unnoticed by the mainstream media in developing or non-‘Western’ countries. Measles is a huge problem in Sub-Saharan Africa and South-East Asia. The USA outbreaks have even been traced to importations from South-East Asia.
OK, but who cares? Isn’t measles a harmless disease of children? My sister had measles as a kid and she was OK.
No not exactly and she’s lucky by the way! Measles can be a very serious illness, which is why we have a vaccine against it. Measles virus doesn’t just cause ugly red spots to form. Measles virus infection is rarely asymptomatic and along with its characteristic rash, symptoms include a fever, cold-like symptoms; immunosuppression and potential nervous system disease.
Complications occur in about a third of measles cases, many of which require hospitalisation. These include diarrhoea, pneumonia, hearing loss and even swelling of the brain. Some of these, such as pneumonia, are linked to the immunosuppression induced by measles. Sadly, 1 in 500 measles cases are fatal.
But we have a vaccine, right?
Yes, and it’s a pretty good one at that: it has an excellent safety record, generates a very good immune response, which is protective, and can be administered along with other vaccines in childhood, like those for mumps, rubella and chickenpox.
But then there are those people who can’t get vaccinated because of [an]other illness, such as cancer; if they contract measles then they are in serious trouble and have a much higher risk of developing complications that are potentially fatal. And young babies can’t get vaccinated either.
This is why herd immunity is so important for such a contagious and serious virus like measles virus.
What’s herd immunity? Sounds like something to do with cattle.
Herd immunity is the non-direct protection from infectious disease, like measles, acquired by rendering a large proportion of a population immune to a disease, which prevents that virus from spreading through the population. If you stop it from spreading, then there is less chance that a person who isn’t protected will come into contact with the virus and get sick. Herd immunity provides additional protection at the level of the population ,which is important for those who cannot get vaccinated and who are most vulnerable to infection.
For a virus that as highly-contagious as the measles virus we need a population immunity level of about 95%, which is very difficult to achieve. Even in highly-vaccinated populations such as the USA and Europe there remain pockets of low vaccination coverage where herd immunity fails and outbreaks flair up and take hold, often with deadly consequences.
If measles is so serious and we have a vaccine that can protect you, then why aren’t people vaccinating by choice?
A few reasons: mostly on ethical or religious grounds, which are not usually based on science. Since the mid-1990s, one of the most common grounds of avoiding the measles vaccine is when used as a component of the measles, mumps and rubella (MMR) vaccine (or the MMRV, a version that includes a vaccine against the virus that causes chickenpox and shingles) has been its apparent association with autism. However, despite rigorous testing this idea is not correct and there is no link between measles vaccines and development of autism.
It is [now] extremely rare that the vaccine is considered unsafe, and usually for sound scientific reasons, such as a child being too sick to receive the vaccine safely, or those people with impaired immune systems or receiving cancer treatment. The vaccine can even be administered to those with an egg allergy, despite being produced using chicken cells. In the U.S.A not vaccinating is referred to [as] ‘philosophical exemption‘ and some states allow parents to forgo vaccinations; other states require children to be up to date with their vaccines before starting public school. Contrary to that, vaccination is not required by law in the UK.
What can we do make people vaccinate more?
More people have to realise that measles can be serious and that we have developed a very safe and very effective vaccine. It is difficult to communicate the seriousness of measles when it is so rare now. The vaccine has become a victim of its own success.
So in summary: education; education; education. But in the absence of that, stronger legislation might help.
Of course, this only works in countries where measles outbreaks are caused by vaccine exemptions. This isn’t the case in Sub-Saharan Africa or South-East Asia. They desperately want the vaccine but often can’t get access to it because of supply problems, like disruption to the cold chain.
This all sounds great – why don’t we have more vaccines like measles for other viruses, like hepatitis C virus, HIV or herpes simplex virus?
That’s a hard one. One of the reasons is that we don’t know why the measles vaccine works so well. We know it is safe; we know it can generate a protective immune response, but we don’t really know how. Scientists are trying though. Without knowing a mechanism it is challenging to ‘reverse engineer’ another virus vaccine based on the measles vaccine. This is the case for many current vaccines, such as the mumps vaccine, yellow fever vaccine and polio vaccine. But there’s nothing to say that what works for measles will work for any other virus. Herpes simplex and hepatitis C viruses are likely to require specialised immune mechanisms.
So if everyone starts to vaccinate more, will we ever eradicate measles?
Probably. It’s a good candidate. It only infects humans, so there’s no animal reservoir; we have an excellent vaccine against it; and it’s easy to spot and report someone who has been infected because its symptoms are so obvious. This is similar to smallpox in humans and rinderpest in cattle, both viruses that have been eradicated previously. Measles will likely follow polio to be eradicated in 21st century but only if we achieve higher rates of vaccination, globally.
But do we really want to eradicate measles virus? Evidence exists that the immunity provided by measles infection and vaccination is protecting us from infection from the close cousins of measles that live in dogs and other mammals, viruses similar to canine distemper virus. But that’s a story for another day. Even after measles is eradicated we will still have to vaccinate.
By Connor Bamford