Vaccination with a single dose of measles containing vaccine at between 9-12 months of age achieves effective immunization rates of between 85-93% Uzicanin 2011. Thus, a second dose of MCV is critical to achieve the commonly cited herd immunity threshold of 95%. Supplemental Immunization Activities (SIAs), which vaccinate
children within specified age groups over a short period of time irrespective
of previous vaccination history, are a common strategy for providing a second
dose of MCV in low- and middle-income countries. SIAs have been a critical part of the Global
Polio Eradication Initiative, and were instrumental in the elimination of
measles and rubella from the Americas Andrus 2011. In the case of measles SIAs,
the Pan American Health Organization employed an operational policy of “catch-up”
campaigns, that targeted children between 9 months and 14 years of age,
followed by subsequent “keep-up” campaigns that targeted children between 9
months and 5 years of age every 3-4 years afterwards. SIAs for measles have
been used in the African region since 1995.
In principle, more frequent campaigns, targeting wider age classes,
while inherently redundant, will necessarily result in higher population
immunity. However, given the costs of
these campaigns, there are incentives to identify the minimal effective
frequency and age targets. Lessler et al LESSLER 2010 used dynamic models to compare the projected levels of population
immunity achieved by vaccination programs that deliver a second dose through
routine immunization to those that use punctuated SIAs. The found that, while
population immunity necessarily declines in the intervals between SIAs, that
SIAs in combination with routine first dose could achieve and maintain
population immunity above the classically cited 92-95% herd immunity threshold
necessary for measles elimination. Similarly, Verguet et al Verguet 2015 highlighted
that the interval between campaigns necessary to maintain target levels of
population immunity are dependent on both the local birth and routine
vaccination rates, as these determine the rate of increase of the susceptible
fraction in between campaigns.