Two methods were used to estimate the number of catch-up vaccinations administered to children aged 15–59months during the observation period of April 2010 to June 2011; age 15–59months was selected during the model design phase before the ACIP provided its recommendation of 14–59months. 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 15–59month cohort were estimated to have received a catch-up vaccination dose during the 15-month observation period [unpublished data].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 15–59months during the observation period versus the control period, comprising the average claims in each month over the prior 2years (i.e. April 2008 through March 2010) [unpublished data].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an increase in office visits for vaccination of catch-up eligible children during the observation period of 24% [unpublished data].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 14–59months, 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 15–59months in March of 2010) to be 39% (6.4 million children).

We estimated cases of IPD, PNE and AOM, and deaths in children aged ≤59months 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%).

In sensitivity analyses, when assuming no indirect-effect acceleration (i.e. 7years until the accumulation of full indirect effects), the current catch-up program is estimated to prevent an additional 0.5 million cases of disease over 10years in children aged ≤59months compared with routine vaccination with PCV13 and no catch-up program (Table ​(Table2).2). Additional uptake of the catch-up program to 4-dose compliance levels is estimated to prevent an additional 0.7 million cases of disease over 10years compared with current catch-up levels.

Our findings are consistent with the previous analysis which examined disease reduction across all ages [3]. The catch-up program’s estimated cost-effectiveness is unchanged by the percentage of children receiving catch-up; therefore, the cost-effectiveness results, although not discussed in the paper, are also consistent with the previous analysis in that the program remains cost-saving if indirect effects are accelerated by ≥6months [3].

Under any scenario of catch-up coverage, infants continue to receive high levels of primary series coverage; therefore, we assume that indirect effects caused by PCV13 will be similar to the experience following PCV7. The previous analysis assumed that the indirect effects due to PCV13 would go through a 7-year ramp-up period and that the increased population coverage of the PCV13 catch-up program would accelerate this ramp-up such that the full indirect effects would occur 1year faster [3]. It is possible that with lower coverage of children aged 15–59months this acceleration could be muted. This assumption has implications on the reduction of disease in all non-vaccinated cohorts in the early years of PCV13. If catch up is insufficient to accelerate the indirect effect, there is still an increase in the number of cases of pneumococcal disease prevented as cases are reduced in those children who do receive the catch up dose. Most notably, if acceleration of indirect effects occurs with 87% participation, but not with 39% participation, the model estimates an additional 2,263 IPD cases, 115,807 PNE cases, 1.7 million AOM cases, and 95 deaths would occur over 10years in children aged ≤59months.