After a disease outbreak is over does that mean the germs have gone?

Dr. Tara C Smith, Professor of Epidemiology, Kent State University College of Public Health

“Not usually,” says infectious disease researcher Tara C Smith, PhD, in her new essayThe Unforgiving Math That Stops Disease. Take, say, measles, mumps, or rubella. After a large outbreak, the viruses will linger, but the level of immunity in the population is high because most susceptible individuals have been infected and (if they survived) developed immunity. Consequently, the viruses spread slowly.

Meanwhile, Smith explains, new susceptible children are born into the population. Within a few years, the population of young children who have never been exposed to the disease dilutes the overall immunity in the population to a level below what’s needed to keep outbreaks from occurring. The virus can then spread more rapidly, resulting in another epidemic, often in 5 to 10-year intervals.

From the bugs point of view, they’re simply lying in wait until there’s a sufficient number of susceptible children around so they can have a breakout year. In the case of measles, each infected child will infect 12 to 18 others.

So is there some way to protect susceptible children? Yes: we can vaccinate them. Furthermore, with measles, if about 95% of the population had immunity to the virus – acquired either through vaccination or having survived the disease – we would also be able to protect those who can’t be vaccinated due to infancy, illness, or old age.

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