The previous analysis assumed that 87% of eligible children would receive a catch-up dose during the first year of the PCV13 vaccination program [3
], equivalent to the percentage of eligible children expected to complete the primary pneumococcal conjugate vaccination series [6
]. For the present analysis, the original model was adapted to include the estimate of actual uptake of PCV13 catch-up [3
]. To maintain consistency with the previous analysis all other model inputs remained as originally reported [3
]. Briefly, the decision-analytic model is a Markov (state-transition) model with the starting point being the choice of vaccination strategy [3
]. The 10-year model considers the entire US population, and allows for inclusion of indirect effects. Within each annual cycle, persons are subject to pneumococcal disease, including IPD, all-cause pneumonia (PNE; hospitalized or non-hospitalized), and all-cause acute otitis media (AOM). During each cycle, persons may survive, die from pneumococcal disease, or die from other causes.
Two methods were used to estimate the number of catch-up vaccinations administered to children aged 15–59
months during the observation period of April 2010 to June 2011; age 15–59
months was selected during the model design phase before the ACIP provided its recommendation of 14–59
months. With the first method, catch-up vaccinations in both private and public markets were estimated from the growth in shipments of PCV13 net of customer inventory changes versus historical shipments made in support of the primary PCV7 series. Product wastage was not considered in the model. From this evidence, approximately 24% (3.8 million children) of the age 15–59
month cohort were estimated to have received a catch-up vaccination dose during the 15-month observation period [unpublished data].
A second method was used to verify the private market component results of the first method: private market only catch-up vaccinations were estimated from growth in SDI insurance claims (SDI claims only capture the private market) for children aged 15–59
months during the observation period versus the control period, comprising the average claims in each month over the prior 2
years (i.e. April 2008 through March 2010) [unpublished data]. Insurance claims for office visits within the catch-up eligible cohort were stable over the control period, thus providing a good baseline for comparison. Claims data from SDI (private market only), also predicted an increase in office visits for vaccination of catch-up eligible children during the observation period of 24% [unpublished data].
In order to project the total uptake of catch-up over the eligible period ending in December 2013, a separate model was developed. Using observed increases of vaccination in children aged 14–59
months, age-specific estimates of catch-up participation were obtained via a financial forecasting model. Briefly, the catch-up eligible population at PCV13 launch was based on census birth estimates and National Health Interview Survey-reported 4-dose compliance for PCV7. As time progressed in the model, children were removed from the target population when they became age-ineligible or were vaccinated with a catch-up dose. Age-relevant ‘well’ visit rates and catch-up vaccination rates were extrapolated to estimate the future catch-up uptake of the remaining eligible population. We assumed that as the catch-up eligible cohort ages over time, their probability of receiving a catch-up dose diminishes due to an inverse relationship between age and both vaccination rates and recommended well-visit compliance. The model utilized age-specific SDI claims data applied to the catch-up eligible population [unpublished data]. This model projected final uptake of catch-up among eligible children (aged 15–59
months in March of 2010) to be 39% (6.4 million children).
We estimated cases of IPD, PNE and AOM, and deaths in children aged ≤59
months over a 10-year time horizon assuming different scenarios of uptake levels of the catch-up program. Additionally, we calculated cases of disease and deaths avoided due to catch-up at the present level and those which could be further eliminated with additional uptake of catch-up to 4-dose compliance levels (87%).
The previous analysis assumed that implementation of the catch-up program would accelerate accumulation of indirect effects [3
], such that full indirect effects would be realized at 6
years as opposed to the 7
years observed with the introduction of PCV7 vaccination [M.M. Moore: unpublished data]. As we estimated that the lower levels of catch-up vaccination may be inconsistent with the strong acceleration of herd effect assumed in the previous analysis [3
], we examined the impact of modifying the assumption of 1-year acceleration through sensitivity analyses. We conducted analyses assuming both 1-year and no acceleration of indirect effects in children aged ≤59
months with the catch-up program, and reported the impact of these assumptions.