Several studies have highlighted the effects of combination vaccines on immunisation coverage at the national or subnational level. This study examined the effects globally. Worldwide introduction of whole-cell pertussis pentavalent (wP-pentavalent) allowed estimation of incremental coverage effects of combination vaccines on the third doses of diphtheria, tetanus, pertussis (DTP3); hepatitis B (HepB3) and type B (Hib3).
Multicountry panel data analysis.
Country-level vaccine coverage data of WHO/UNICEF for the years 1980-2018.
Linear mixed models were used to estimate the effects of wP-pentavalent introduction by incorporating proxy variables to control for time trend and other time-dependent changes in the immunisation programmes.
Introduction of combination vaccines may have improved the coverage of DTP3 by 3percentage points(95% CI 2.5% to 3.6%) globally compared with the coverage in the pre-combination vaccine era. The comparison of coverage rates of HepB3 and Hib3 in before and after wP-pentavalent periods indicates that the introduction of combination vaccines improved the coverage by 10.1 percentage points (95% CI 8.4% to 11.7%) for HepB3 and 9.9 (95% CI 7.1% to 12.7%) for Hib3 in countries that introduced those antigens prior to adoption of wP-pentavalent. Even though the incremental coverage increase of DTP3 appears quite modest, it is still a significant result, especially because DTP vaccine has been in the national immunisation programmes of all countries for about 24 years prior to the introduction of wP-pentavalent. Additionally, the introduction of pentavalent also allowed inclusion of Hib and HepB in the vaccine schedule for a large number of countries (85 and 37, respectively, of the 102 countries included in our analysis).
The findings suggest that development of combination vaccines with additional antigens is likely to help sustain and improve coverage of existing as well as new childhood vaccines.

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