Assessing the impact of heat waves on childhood immunization coverage in Sindh, Pakistan: Insights from 132.4 million doses recorded in the provincial electronic immunization registry (2018-2024).

INTRODUCTION: Heat waves, intensified by climate change, are increasingly challenging health systems, particularly in low- and middle-income countries (LMICs). Pakistan, ranked among the top 10 most climate-vulnerable nations, faces significant challenges in maintaining routine immunization coverage rates amid soaring temperatures. This study examines the impact of heat waves on immunization delivery in Sindh Province of Pakistan, a region highly vulnerable to climate-induced disruptions.

METHODS: We analyzed child-level data from the provincial electronic immunization registry for 132.4 million immunization doses administered between January 1, 2018, to July 31, 2024. We used the clustered panel univariate and multivariable Poisson and negative binomial regressions to analyze the association between high temperature alert days (33 °C to 39.9 °C) and heat waves (> 40 °C) and immunizations, by vaccination modality (fixed site, routine outreach, enhanced outreach). The analysis controlled for external shocks, such as floods, the COVID-19 pandemic, and vaccinators' strikes, and accounted for geographic and temporal variation.

RESULTS: Heat waves and high temperature alert days (≥33 °C) significantly reduced immunizations, with routine and enhanced outreach activities being most affected (13.6 % and 21.2 % decline respectively). Fixed-site immunizations witnessed a comparatively lower decline i.e. 5.8 %. Rural Divisions Larkana and Sukkur were disproportionately affected, while Karachi exhibited minimal impact. Despite the negative impact of heat waves, immunization efforts intensified during external shocks like floods and the COVID-19 pandemic, particularly through prolonged and frequent outreach activities.

DISCUSSION: Heat waves disrupted vaccine service delivery leading to reduced immunization coverage in Sindh, disproportionately affecting immunizations administered through outreach activities. Geographic and temporal variations highlight the need for localized strategies, including improved infrastructure, optimized outreach schedules, and robust vaccine cold chains. Future research should explore long-term adaptive strategies for maintaining vaccination coverage amid increasing impact of climate change, especially in low-resource settings.