The classical formulation of the catalytic model assumes that the
force of infection, whether age dependent or not, is constant over time
Griffiths 1974. Whitaker and Farrington
Whitaker 2004 illustrated that the estimates of the classic method were robust
to the annual and multi-annual cyclic fluctuations commonly seen for many
childhood infections. However, in the era of vaccination control, we have
observed large directional changes in prevalence of many infections; e.g. a 78%
reduction in measles cases between 2000-2012 Simons 2012, the elimination of
rubella from the Americas [REF-PAHO statement ]. The basic principles of the catalytic model
suggest that, even in the absence of reporting of total cases, these declines
in disease prevalence should be reflected in the changing age distribution of infection
or sero-prevalence. Though long-term age-specific surveillance is rare for most
infections, Ferrari et al Ferrari 2013 performed a comparative analysis of the age
distribution of measles cases between 2001-10 to quantify the impact of
increased vaccination on measles force of infection. Over that time period
there was a strong correlation between first dose measles vaccination coverage and
the mean age of infection. The slope of
this relationship was mediated by the birth rate, with vaccination having
smaller impact on the mean age of infection in high birth rate settings. This
comparative analysis suggests the pattern that is expected to be observed as
countries continue to increase vaccination coverage over time.