ACIP Recommends Adults Receive Seasonal, Single-Dose Influenza Vaccines Free of Thimerosal

The Advisory Committee on Immunization Practices (ACIP) voted 5-1 with 1 abstention to recommend that all US adults receive seasonal influenza vaccines in single-dose formulations that do not contain thimerosal. The committee also specifically voted, in the same margin, to recommend that children 18 years and pregnant women also receive a single dose of thimerosal-free influenza vaccines.

The Advisory Committee on Immunization Practices meeting took place at the CDC campus in Atlanta, GA. | Image Credit: © Tada Images - stock.adobe.com

Prior to the votes regarding thimerosal, ACIP voted 6-0, with 1 voter abstaining, to reaffirm the recommendation that all persons aged 6 months and older who do not have contraindications receive routine annual influenza vaccination.

Thimerosal and Influenza Vaccines

Thimerosal is a preservative containing mercury mostly used in multi-dose vaccines to prevent contamination. It was used in vaccines for decades but phased out of childhood immunizations in 2001 out of an abundance of caution due to questions regarding toxicity and possible connections to childhood neurodevelopmental disorders, including autism. A litany of studies—including a review published days ago by the CDC, which was subsequently removed from their website—have affirmed that no such link exists.^1-3

Though an exception in thimerosal use remains in the form of a small group of multi-dose influenza shots that can be administered for adults or children, the CDC says that 96% of all influenza vaccines in the US administered in the 2024-2025 season did not contain thimerosal. This rate decreases further, to 0.3%, when considering just pregnant women. The CDC writes on their website that thimerosal use in vaccines has a superb safety record, with data from many studies indicating no harm associated with low doses of thimerosal in vaccines.^1-3

H. Cody Meissner, MD, professor of pediatrics at Dartmouth Geisel School of Medicine, was the lone voting member who rejected the thimerosal influenza vaccine recommendations. He explained his concerns that the new guidance could inadvertently make it harder to access vaccines for the most vulnerable populations, especially in time for next respiratory season. Meissner also cited the apparent thimerosal in vaccines, arguing that there was no need for ACIP to relitigate the issue.⁴

"The risk from influenza is so much greater than the nonexistent, as far as we know, risk from thimerosal," Meissner said. "I would hate for a person not to receive the flu vaccine because the only available vaccine contains thimerosal."⁴

New RSV Recommendations for Infants

The meeting, chaired by Martin Kulldorff, PhD, a biostatistician and epidemiologist who was formerly a professor of medicine at Harvard University, took place over the course of 2 days and covered other respiratory diseases, including respiratory syncytial virus and COVID-19.

For RSV, members heard presentations regarding maternal and pediatric RSV vaccines, including the maternal RSV vaccine (Abrysvo; Pfizer) and, for infants aged 8 months or younger, the antibody immunization nirsevimab (Beyfortus; Sanofi, AstraZeneca). These candidates are approved by the FDA and recommended by ACIP for use in their respective populations, and the data presented to ACIP reaffirmed their effectiveness.

RSV continues to have a severe burden on infants, necessitating novel prevention methods that can be used to prevent the worst of the disease. Recently, the FDA approved clesrovimab-cfor (Enflonsia; Merck) for the prevention of RSV-related lower respiratory tract disease in newborns and infants who are born during or entering their first RSV season. Clesrovimab is a long-lasting, preventive monoclonal antibody that can provide protection over the course of a single RSV season.⁵

The committee was shown data indicating the effectiveness of clesrovimab in clinical trials. Based on the encouraging data and FDA approval, ACIP voted 6-0, with 1 abstention, to recommend clesrovimab for infants aged less than 8 months born during or entering their first RSV season who are not protected by maternal vaccination. Pending final approval, clesrovimab will join nirsevimab as monoclonal antibodies recommended for RSV respiratory tract protection, though only nirsevimab is recommended for children aged 8 to 19.

“Ahead of the 2025-2026 RSV season, we are proud to offer [clesrovimab] as a new preventive option designed to protect healthy and at-risk infants from RSV disease across a spectrum of severity, including worsening disease requiring hospitalization,” Richard M. Haupt, vice president and head of global medical and scientific affairs at Merck Research Laboratories, said in a news release. "The committee’s recommendation is an important step forward in efforts to help reduce the significant burden RSV continues to place on infants, families, and health care systems.”⁶

COVID-19 Epidemiologic and Vaccine Discussions

Kulldorff began the meeting by affirming the independence of ACIP and their mission to issue evidence-based recommendations that parents and caregivers can trust, with "no questions off limits." He decried coverage of the new members of ACIP, appointed by Health and Human Services Secretary Robert F. Kennedy Jr. after he removed the 17 sitting members of the committee. Kulldorff said the questioning fed into the already festering vaccine hesitancy across the country. He further attributed the hesitancy—particularly feelings drummed up since the pandemic—in part to "the inflated promises of the COVID-19 vaccines preventing transmission and severe disease" and COVID-19 vaccine mandates.⁷

"Messages stemming from evidence-based science on the safety and efficacy of vaccines are in the best interest of the public, industry, and public health institutions," Kuldorff said.⁷

Following this introduction, updated data on the epidemiology of COVID-19 was presented to the committee. The data contradicted the sentiment of Kuldorff's opening statement, indicating that COVID-19 remained a serious public health threat and cause of death in the US and that COVID-19 vaccines continued to be safe and effective at preventing hospitalizations.

Interestingly, data highlighted that COVID-19 could cause severe disease in young children and in pregnant women but that vaccines can provide them robust protection. These insights are at odds with a recent directive from Secretary Kennedy and the CDC to no longer recommend that young children and pregnant women receive annual COVID-19 vaccination, though the discussion to receive one could still be held with their doctor.

REFERENCES

1. CDC. Thimerosal-containing vaccines and neurodevelopmental outcomes—Review of the evidence. Published June 23, 2025. Archive Accessible Online via Wayback Machine: https://web.archive.org/web/20250624164828/https://www.cdc.gov/acip/downloads/slides-2025-06-25-26/Thimerosal-Containing-Vaccines-Summary-of-Evidence-508.pdf. Accessed June 26, 2025.

2. Associated Press. What to know about thimerosal, vaccine preservative from bygone flu-shot debate. PBS News. Published June 26, 2025. Accessed June 26, 2025. https://www.pbs.org/newshour/health/what-to-know-about-thimerosal-vaccine-preservative-from-bygone-flu-shot-debate

3. CDC—Influenza (Flu). Thimerosal in flu vaccine. Last Updated September 17, 2024. Accessed June 26, 2025. https://www.cdc.gov/flu/vaccine-safety/thimerosal.html

4. CDC. Advisory Committee on Immunization Practices (ACIP) - Day 2 of 2. YouTube. 4:59:02. Streamed June 26, 2025. Accessed June 26, 2025. https://www.youtube.com/live/z-16fImZoEc

5. Ferruggia K. Clesrovimab receives FDA approval for RSV lower respiratory tract prevention in infants. Pharmacy Times. Published June 9, 2025. Accessed June 26, 2025. https://www.pharmacytimes.com/view/clesrovimab-receives-fda-approval-for-rsv-lower-respiratory-tract-prevention-in-infants

6. Merck. ACIP Recommends Use of Merck’s ENFLONSIA™ (clesrovimab-cfor) for Prevention of Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease in Infants Younger than 8 Months of Age Born During or Entering Their First RSV Season. News Release. Released June 26, 2025. Accessed June 26, 2025.

7. CDC. Advisory Committee on Immunization Practices (ACIP) - Day 1 of 2. YouTube. 17:50:00. Streamed June 25, 2025. Accessed June 26, 2025. https://www.youtube.com/live/2PnYYb_xj3U?list=TLPQMjYwNjIwMjUtNg09onWj8Q&index=3