Thimerosal in Vaccines

What is Thimerosal?

• Definition: A mercury-based compound used as a preservative in vaccines.

• Composition: Contains ethylmercury, which is different from methylmercury (toxic form found in fish/industrial waste).

• Function: Prevents bacterial and fungal contamination in multi-dose vials, especially after repeated use.

History & Usage

• Introduced in the 1930s in vaccine manufacturing.

• Phased out in many countries (e.g., USA) in early 2000s due to public concern, not scientific proof of harm.

• Still used in India’s routine immunisation programme, particularly in multi-dose vials of DTP and Hepatitis B.

Scientific Evidence on Safety

• No credible evidence linking thimerosal to autism or developmental delays.

• Major studies:

  • 2004: US Institute of Medicine reviewed 200+ studies—found no link with autism.

  • CDC and FDA confirmed no harmful effects.

• Expert View: 

  • “Decades of data show no link to autism or serious toxicity.”

  • Autism rates did not decline after thimerosal removal in some countries.

Ethylmercury vs. Methylmercury

• Thimerosal-free (single-dose) vaccines are used in private sector.

• Public immunisation uses multi-dose vials (cost-effective, storage constraints).

• Thimerosal helps maintain vaccine safety in mass immunisation settings.

Adverse Reactions

• Minor: Redness, swelling at injection site, mild allergic reactions.

• Serious: Extremely rare.

Current Debate

• Triggered by global concerns and misinformation (e.g., linking mercury in vaccines to autism).

• US committee recently voted against recommending flu vaccines with thimerosal, potentially undermining vaccine confidence.

• Thimerosal is safe at approved levels.

• No link to autism as per global scientific consensus.

• Public awareness and consulting doctors are key to vaccine confidence.