Vaccines can save children with non-preventable diseases – Authors' reply

Senjuti Saha and Samir Saha make the excellent point that the scope of vaccine impact goes beyond the deaths that are directly averted by immunisation activities. In this we agree. We aimed to quantify the deaths averted by vaccination for ten diseases in 98 low-income and middle-income countries.¹ However, there are wider benefits of vaccination—for example, in reducing the burden on health-care services. With many low-income and middle-income countries having minimal health-care capacity, the impact of vaccination might far outstrip the current best estimates.

COVID-19 has emphasised the ramifications of a health-care capacity that is resource-limited as countries have seen the pandemic saturate possible treatment space, with the introduction of vaccines relieving some of this burden. We noted that vaccination activities reduced overall mortality by 45% between 2000 and 2019, for the countries and pathogens that were studied. However, these data do not capture the reduction in morbidities that are associated with vaccine-preventable diseases, nor the benefits of strengthened health systems and equity.

Quantifying the wider effects of vaccination has been attempted, particularly in an economic sense. Chang and colleagues² assessed the role of vaccination in reducing medical impoverishment, noting a 9% reduction in the number of people in low-income countries whose income is below the World Bank poverty line. The burden of vaccine-preventable diseases disproportionately affected the lowest income quintiles, showing the potential equalising nature of vaccination activities. Vaccination has also been linked with productivity, shown by a measurable improvement in cognitive outcomes in later childhood.³ Improved educational achievements are generally linked with increased social mobility and economic development.⁴

Given the wide-ranging impact of vaccination, why are epidemiological and health economic studies so focused? In our study, 16 modelling groups provided estimates that considered heterogeneity in data, transmission, and health access both geographically and, in some cases, temporally. To provide robust, well calibrated estimates of disease burden, focused analysis is required. This requirement motivates the structure of the Vaccine Impact Modelling Consortium, which brings together modelling groups to capture the latest insights in disease transmission and vaccine impact. Despite this motivation, uncertainties remain in both the natural history of the pathogens that are studied and the input data relating to vaccination and demography. As such, we need to balance the need for robust and focused results, data scarcity, and the huge and far-reaching ramifications of such an effective intervention as vaccination. It is an area of continued study and improvement that will potentially be accelerated by the ongoing COVID-19 pandemic.

KAMG, JT, SE-L, XL, and NMF report grants from the UK Medical Research Council, Bill & Melinda Gates Foundation, and Gavi, the Vaccine Alliance, during the conduct of the study. NMF reports grants from the National Institutes of Health National Institute of General Medical Sciences, UK National Institute of Health Research, and Janssen Pharmaceuticals, unrelated to this Correspondence.