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Population health

Incomplete vaccine coverage increases the risk of disease for the entire population, including those who have been vaccinated, because it reduces herd immunity. For example, the measles vaccine is given to children 9–12 months old, and the window between the disappearance of maternal antibody and seroconversion means that vaccinated children are frequently still vulnerable. Strong herd immunity reduces this vulnerability. Increasing herd immunity during an outbreak or when there is a risk of an outbreak is perhaps the most widely accepted justification for mass vaccination. When a new vaccine is introduced, mass vaccination can help increase coverage rapidly.

If enough of a population is vaccinated, herd immunity takes effect, decreasing risk to people who cannot receive vaccines because they are too young or old, immunocompromised, or have severe allergies to the ingredients in the vaccine. The outcome for people with compromised immune systems who get infected is often worse than that of the general population.