It is important to point out, and for everyone to remember, that the anti-vaccine movement is not just misguided, they are objectively wrong. It is especially important while they are gearing up to “prereject” any possible COVID-19 vaccine. One thing for which there is strong expert agreement regarding the pandemic is that we won’t really get it under control until we have an effective vaccine. But the effectiveness of that vaccine will depend largely on public acceptance, which makes antivaxxers a danger to the public health, and as we are seeing, the global economy.

A recently updated Cochrane systematic review of the safety and effectiveness of the MMR vaccine is a good time, therefore, to emphasize that antivaxxers have been wrong about the MMR vaccine for over two decades. They claim it doesn’t work, and that it isn’t safe. They tried to link it to autism (and when that failed they migrated over to the preservative thimerosal, and when that failed they just blamed the vaccine schedule and vaccines in general). They are persistently and demonstrably wrong, which of course never slowed down an ideologically motivated activist, and certainly not a conspiracy theorist.

This new review shows that the evidence for the safety and effectiveness of the MMR vaccine is overwhelming. The authors report:

We included 138 studies (23,480,668 participants). Fifty‐one studies (10,248,159 children) assessed vaccine effectiveness and 87 studies (13,232,509 children) assessed the association between vaccines and a variety of harms.

That is a lot of data – over 23 million participants. These include studies from around the world, by many independent researchers and organizations. That is a lot of replication.

Regarding effectiveness, we have to consider each component of this trivalent vaccine separately. Sometimes the research looked at the MMRV vaccine – mumps, measles, rubella, and varicella, so there is data on all four components. Regarding measles they report:

Vaccine effectiveness in preventing measles was 95% after one dose…and 96% after two doses. The effectiveness in preventing cases among household contacts or preventing transmission to others the children were in contact with after one dose was 81%…after two doses 85%…and after three doses was 96%.

(I removed the technical statistical details for readability, but you can get that from the original report.) So after all three doses of MMR, children were 96% protected from measles, and this also prevented 96% of spreading of the virus. This is important, and a main reason for the third dose. It also highlights the fact that these are two different things, preventing infection and preventing spread. There is a lot of discussion about antibodies in those with COVID-19, and it is important to remember that we still don’t know how effective those antibodies will be in preventing further spread vs illness.

The numbers for preventing illness were similar for the other components – mumps 86%, rubella 89%, and varicella 95%. Protection is long term, lasting years and perhaps for life, but the authors point out that we do need more data on the very long-term effects of the vaccine.

What about safety? There is evidence for an association between getting the vaccine and risk of febrile seizures, but this is small. Overall there is a 2-4% risk of healthy children getting a seizure with a fever before the age of 5. With the vaccine there is an extra case every 1,150-1,700 vaccinations, which is relatively small. They also report:

The analyses provide evidence supporting an association between MMR vaccination and idiopathic thrombocytopaenic purpura (ITP). However, the risk of ITP after vaccination is smaller than after natural infection with these viruses. Natural infection of ITP occur in 5 cases per 100,000 (1 case per 20,000) per year. The attributable risk is estimated about 1 case of ITP per 40,000 administered MMR doses.

This is also an important general point – yes there is risk with the vaccine, but the risk with getting infected is even higher. In the case of ITP getting infected with one of these diseases is twice as likely to be associated with ITP than the vaccine. This is a good demonstration of why you cannot only look at risk when it comes to any medical intervention. That is a classic mistake of anti-vaccine rhetoric. You must look at risk vs benefit.

Finally, the reviewers report that after more than 13 million children who received the MMR were followed, there was no association between the vaccine and autism, cognitive delay, encephalitis or encephalopathy. Does this prove a zero risk? No. No amount of data ever will. But it demonstrates convincingly that the risk, if not actually zero, is indistinguishable from zero. It is so small that it is in the background, less than a million to one, and not worth worrying about. Again, the risk from the diseases that the vaccine prevents must be greater by many orders of magnitude.

Bottom line – the MMR vaccine is safe and effective. The data clearly show this, and yet scientists continue to study and monitor this vaccine and gather more data. This is reassuring, but the antivaccine movement has proven immune to facts and reality. If someone insists on being so at odds with established medical facts, the best response is to marginalize them. This is no more true during a pandemic, but is more acute, and hopefully more obvious.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.