Follow-up vaccination campaigns are an essential component of PAHO's measles eradication strategy. A follow-up campaign is defined as a measles vaccination campaign which targets all children 1 to 4 years of age, regardless of prior vaccination status or disease history.

However efficient the catch-up (vaccination campaign aimed at all children 1-14 years of age) and keep-up (vaccination through routine services) vaccination efforts are, there will inevitably be an accumulation of measles susceptible preschool-aged children over time. The primary purpose of follow-up campaigns is to prevent this accumulation of susceptible children from reaching dangerous levels which can increase the risk of a measles outbreak.

Two factors contribute to the build-up of susceptible children. First, measles vaccine is less than 100% effective, thus leaving some children unprotected following vaccination. Second, measles vaccination coverage for each birth cohort will almost always fall short of reaching all children.

PAHO's measles eradication strategy recommends that periodic follow-upvaccination campaigns be conducted whenever the estimated number of measles susceptible preschool-aged children (children 1-4 years of age) approaches the size of an average birth-cohort. The interval between campaigns will depend upon the vaccination coverage obtained among infants through routine services since the last campaign. The lower the average routine vaccination coverage, the shorter the interval between campaigns. For example, if only an average of 60% routine coverage is obtained, a follow-up vaccination campaign would be needed approximately every two years; if 80% average coverage is obtained, then campaigns will be needed approximately every four years. The maximum allowable interval between campaigns is 4 years. Most countries of the Americas are able to maintain an average routine coverage of at least 80% and conduct follow-up campaigns every 4 years.

Measles vaccination activities by country.