On Feb. 4, Seth Johnson asked how an unvaccinated individual is a threat to the vaccinated (“Letter to the editor: Questions remain despite reporting on vaccination”). Let me take a shot at that. To answer, I’ll have to use math – sorry!

Imagine 1,000 people in a room with a person infected with measles. Ninety percent of the susceptible people will get measles. If 5 percent of the 1,000 are unvaccinated, those 50 people will account for 45 new infections.

But the vaccine is only 97 percent effective in its protection – not perfect, but good enough to essentially eliminate the 400 to 500 measles deaths a year in the U.S., pre-vaccine.

So the vaccinated group will have 3 percent susceptible to infection. A 90 percent infection rate in those 28.5 individuals yields an additional 25.5 cases of measles. Both groups will suffer infection: 90 percent of the unvaccinated and close to 3 percent of the vaccinated.

lf the vaccination “opt-out” rate doubles to 10 percent, simple math reveals a 60 percent increase in cases, all in the unvaccinated people.

But that effect is multiplied by the fact that more active cases means more potential exposures in other rooms of 1,000 people – beyond my simple math! Quarantine (from the Italian for “40 days”) has been employed when a critical mass of serious infection is reached.

This is a simplistic answer to Johnson’s question, but it should be noted that he asked the wrong question. From a public health perspective, we should worry not only about those unvaccinated by choice and the 3 percent of the vaccinated in whom immunity is not achieved.

We need to include the additional population that can’t be vaccinated: too young, too ill and others. The “refuseniks” put all of them in peril.

Steven Zimmerman

Topsham

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