Ensuring vaccine cold chain integrity: A rapid and low-cost test for identifying heat-exposed sucrose-containing vaccines.

Maintaining cold-chain integrity is vital for vaccines to ensure that they remain within the recommended temperature limits during routine storage and transportation. This ensures vaccine stability, efficacy, and avoids degradation. Here, we propose rapid and low-cost tests based on simple glucose assays to detect heat-exposed degraded sucrose-containing vaccines through sucrose's inherent gradual conversion to glucose when exposed to elevated temperatures. Bioluminescent and colorimetric assays and a clinical biochemical analyser for urine samples could successfully determine effects of heat exposure on vaccines by detecting a significant increase in glucose levels. We show that this increase in glucose also correlates with the loss of vaccine potency. When vaccines were incubated at 37 and 45 °C, the bioluminescent assay was able to detect an increase in glucose levels from 12 h of heat exposure. The biochemical analyser could successfully detect increased glucose levels in a COVID-19 vaccine which had been exposed to 37 and 45 °C. Most importantly, the colorimetric assay has the advantage of producing a colour change visually upon simply mixing the vaccine with a reagent without the need for a plate reader or any other sophisticated devices. To our knowledge, this is the first simple, rapid and device-free test of its kind to detect heat-exposed substandard vaccines, making it a potential test for deploying at key points in the supply chain in warm and hot countries to check the integrity of vaccine cold-chain. Although this test does not replace the more definitive lot release assays such as potency assays, it could initially be used as a rapid and low-cost test to identify substandard sucrose-containing vaccines within supply chains, in support of WHO's Prevent, Detect, and Respond strategy.