Thimerosal is an ethyl mercury–containing compound that has been used safely for >60 years as a preservative in multidose vials of vaccines to prevent bacterial and fungal contamination of those vials when they are repeatedly entered to withdraw doses.1,2  In the late 1990s, preservative-free single-dose vials were widely introduced into high-income countries (HICs). This was a precautionary move in response to theoretical concerns, now known to be unfounded, that ethyl mercury in thimerosal could build up in vaccine recipients’ bodies at a rate to similar methylmercury (a known toxin) causing toxicity. For low- and middle-income countries (LMICs), where the burdens of vaccine-preventable deaths are most profound, multidose vials of thimerosal-preserved vaccines are a critical part of immunization programs. Extensive additional resources associated with increased manufacturing, shipping, cold-chain storage, administration, and waste-handling infrastructure would be required by a move away from multidose vaccines; for example, a shift...

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