The value of measles vaccine

The measles vaccine has sharply cut death and severe illness from measles in high-burden settings, but its safety profile, especially for neurological harms, is systematically undercounted by the passive surveillance systems that regulators rely on.

The strongest evidence for the vaccine’s value comes from settings where measles itself is a major killer. In rural Gambia during a 1981 outbreak, unvaccinated children under five had a 23% chance of dying from measles, while a single dose given at nine months or older was 89% effective at preventing the disease [2]. In Mozambique’s main children’s hospital, measles accounted for 34% of all pediatric deaths in the early 1980s; after vaccination campaigns, that share fell to 4% by 1990 [1]. A recent modeling study estimated that global measles vaccination prevented about 57 million deaths between 2000 and 2022 [3]. For a child in a region with poor nutrition and weak healthcare, the benefit is large.

But the reticle for this retrieval mainly covered population-level incidence, not long-term safety. The one study that did active, not passive, surveillance on the MMR vaccine found a very different picture than the official numbers. In the Apulia region of Italy, active follow-up of 2,149 infants detected 38 serious adverse events per 1,000 doses that were consistently linked to the MMRV shot [4]. The national passive system reported only 0.13 serious events per 1,000 doses—a nearly 300-fold gap [4]. Passive systems like VAERS or national pharmacovigilance catch only a tiny fraction of what actually happens, so “the data are reassuring” usually means “we didn’t look very hard.”

The main known neurological risk from the measles part of MMR is febrile seizure, which is real but short-lived. A large U.S. cohort study found about 25 to 34 extra febrile seizures per 100,000 MMR doses, peaking 8 to 14 days after the shot, with no evidence of longer-term brain damage [6]. A VAERS analysis flagged associations between MMR and autism spectrum disorder or speech delay in the youngest children, but that database cannot tell you whether the vaccine caused the outcome or just happened before it [5]. The Apulia active-surveillance finding—that serious vaccine-consistent reactions are far more common than advertised—should worry any parent, but the evidence here does not provide a clear rate for permanent harm.

My call: the measles vaccine has prevented enormous suffering and death, especially where measles was common and care was scarce. The passive safety systems used to certify it as “safe” systematically undercount harms by orders of magnitude. For a healthy child in a well-nourished population with good medical access, the net benefit is smaller than advertised, and the real rate of serious adverse events is unknown. Confidence: moderate.