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Background Childhood immunisation coverage has stagnated globally and even declined in some countries during recent decades. We aimed to evaluate whether a public health programme targeted at diarrhoeal disease also impacted, as a spillover effect from the programme, immunisation coverage.Methods Retrospective data from the 2015/2016 Indian Demographic Health Survey were obtained on 142 921 children, born to mothers in the 5 years leading up to the survey. A difference-in-differences study was performed to compare the completed child vaccination rates before and after the Clinton Health Access Initiative Zinc (CHAI)/Oral Rehydration Salts Programme in 2012, and between covered (Madhya Pradesh and Uttar Pradesh) and non-covered states.Results The CHAI programme was associated with an increase in basic child vaccination coverage (one dose of BCG (Bacille Calmette-Guérin) and measles, and three doses of DTP (diphtheria, tetanus and pertussis) and polio vaccines) of 5.27 percentage points (95% CI 3.48 to 7.05). This increase was concentrated among male children, and no differences were found by wealth quintiles, maternal education or sanitation. Different intensities of the CHAI initiative across districts in Uttar Pradesh were not associated with different vaccination coverage rates.Conclusions Existing studies rarely evaluate spillover effects or unintended consequences of public health campaigns. We find that a public health programme to reduce diarrhoeal diseases was associated with increased vaccination coverage in India. These findings highlight the potential horizontal health effects of disease-specific public campaigns and household behaviour responses to the programme. The evidence shows that actionable approaches to improve immunisation coverage exist that do not exclusively rely on vaccination campaigns.