Influenza vaccines and thimerosal
Thimerosal is an ethylmercury-containing preservative that was removed from routinely recommended childhood vaccines by 2001. The preservative is still contained in some doses of influenza vaccine when they are packaged in multi-dose vials.
During the June 2025 meeting of the Advisory Committee on Immunization Practices (ACIP), the newly seated committee members took three votes related to thimerosal in influenza vaccines. In all three cases, they voted against the use of thimerosal-containing influenza vaccines.
To a casual observer, these votes may seem like the new committee is working to make vaccines safer. However, these votes were concerning for several reasons:
- Thimerosal, even when it was in a greater number of vaccines, has not been found to be damaging to health — contrary to the evidence presented during the meeting.
- To prepare for the votes, a single presentation by a self-described “private citizen” was the only information included during the meeting. Setting aside the fact that the presenter did not share important conflicts of interest — and had been a member of an anti-vaccine group — this approach was out of the norm for how ACIP operates in several ways. First, typically a topic will have several presentations, including some by CDC scientists, and any presentation would have been reviewed and approved by CDC scientists prior to inclusion. Second, prior to being added to the agenda, larger groups of stakeholders, known as work groups, would have met for several months to more carefully examine the existing data, ask questions, and hold discussions — essentially turn over every stone to understand and inform the issue. The exception to this process would be if new, concerning data about vaccine safety had come to attention, but that was not the case here. In fact, CDC scientists had put together a 17-page document analyzing the body of literature related to thimerosal. The evidence demonstrated that thimerosal in vaccines was not causing harm — at variance with the presentation given during the meeting. Notably, after briefly being posted on the CDC’s website with other meeting materials, that 17-page document was removed. During the meeting, one of the new committee members indicated that it had been removed because it wasn’t approved for posting by the Department of Health and Human Services. Third, a standardized process for reviewing information is typically completed to ensure nothing is overlooked. This is called the Evidence to Recommendation (EtR) framework. That process was not completed for this topic.
- Unless a pressing safety concern exists, the timing for a change such as this would have been planned in a manner that would ensure vaccine supply was not disrupted. Unfortunately, this consideration did not appear to be part of the calculation as many healthcare providers would have already ordered their influenza vaccine supply for the fall and some providers, particularly those that vaccinate large groups such as in nursing homes, would have likely ordered mostly multi-dose vials. Vials that contain multiple doses of a vaccine are the type affected by this change because vaccines packaged in multi-dose vials contain thimerosal to prevent contamination when later doses are removed from the vial. This late change may result in some individuals being unable to get vaccinated against influenza for the upcoming season. In essence, the lack of understanding of the overall system demonstrated by the new committee members created chaos and uncertainty where they did not need to exist. Hopefully, we will not find out that this incident also caused harm by impacting vaccine access and supply.
The bottom line
Thimerosal was no longer in most vaccines. Evidence that accumulated over two decades did not show it to compromise vaccine safety. No new evidence had emerged to the contrary. The actions of the newly appointed committee caused concerns about the reliability of data that will be used to make decisions, the processes that will be followed, and the ability of the newly appointed committee members to evaluate the scientific data adequately and without bias.
Measles cases rise to highest in 33 years
In 2000, measles was declared eliminated from the U.S. Elimination is “public health jargon” that means the virus was no longer continuously spreading within the U.S. for a period of at least 12 months. Instead, the only cases were isolated occurrences when the virus was introduced through travel. Immunization rates were high enough that the virus could not find enough susceptible people to keep spreading.
Unfortunately, several areas with decreased immunization rates now exist, enabling the virus to continue to spread. We are currently entering our seventh month with continuous spread of measles in the U.S. If cases continue through January 2026, the U.S. will lose its status as a country that has eliminated measles.
More than the loss of a status, it is important to realize that this means people have continued to be infected with measles. Indeed, the number of people infected with measles is greater than any outbreak in the last 33 years. Further, three people in the U.S. have died from measles in 2025, which equals the total number of deaths from measles in the last 25 years combined. The two children who have died from measles this year are the first child deaths from measles in the U.S. since 2003.
Find out more:
Influenza vaccines and thimerosal
Thimerosal is an ethylmercury-containing preservative that was removed from routinely recommended childhood vaccines by 2001. The preservative is still contained in some doses of influenza vaccine when they are packaged in multi-dose vials.
During the June 2025 meeting of the Advisory Committee on Immunization Practices (ACIP), the newly seated committee members took three votes related to thimerosal in influenza vaccines. In all three cases, they voted against the use of thimerosal-containing influenza vaccines.
To a casual observer, these votes may seem like the new committee is working to make vaccines safer. However, these votes were concerning for several reasons:
- Thimerosal, even when it was in a greater number of vaccines, has not been found to be damaging to health — contrary to the evidence presented during the meeting.
- To prepare for the votes, a single presentation by a self-described “private citizen” was the only information included during the meeting. Setting aside the fact that the presenter did not share important conflicts of interest — and had been a member of an anti-vaccine group — this approach was out of the norm for how ACIP operates in several ways. First, typically a topic will have several presentations, including some by CDC scientists, and any presentation would have been reviewed and approved by CDC scientists prior to inclusion. Second, prior to being added to the agenda, larger groups of stakeholders, known as work groups, would have met for several months to more carefully examine the existing data, ask questions, and hold discussions — essentially turn over every stone to understand and inform the issue. The exception to this process would be if new, concerning data about vaccine safety had come to attention, but that was not the case here. In fact, CDC scientists had put together a 17-page document analyzing the body of literature related to thimerosal. The evidence demonstrated that thimerosal in vaccines was not causing harm — at variance with the presentation given during the meeting. Notably, after briefly being posted on the CDC’s website with other meeting materials, that 17-page document was removed. During the meeting, one of the new committee members indicated that it had been removed because it wasn’t approved for posting by the Department of Health and Human Services. Third, a standardized process for reviewing information is typically completed to ensure nothing is overlooked. This is called the Evidence to Recommendation (EtR) framework. That process was not completed for this topic.
- Unless a pressing safety concern exists, the timing for a change such as this would have been planned in a manner that would ensure vaccine supply was not disrupted. Unfortunately, this consideration did not appear to be part of the calculation as many healthcare providers would have already ordered their influenza vaccine supply for the fall and some providers, particularly those that vaccinate large groups such as in nursing homes, would have likely ordered mostly multi-dose vials. Vials that contain multiple doses of a vaccine are the type affected by this change because vaccines packaged in multi-dose vials contain thimerosal to prevent contamination when later doses are removed from the vial. This late change may result in some individuals being unable to get vaccinated against influenza for the upcoming season. In essence, the lack of understanding of the overall system demonstrated by the new committee members created chaos and uncertainty where they did not need to exist. Hopefully, we will not find out that this incident also caused harm by impacting vaccine access and supply.
The bottom line
Thimerosal was no longer in most vaccines. Evidence that accumulated over two decades did not show it to compromise vaccine safety. No new evidence had emerged to the contrary. The actions of the newly appointed committee caused concerns about the reliability of data that will be used to make decisions, the processes that will be followed, and the ability of the newly appointed committee members to evaluate the scientific data adequately and without bias.
Measles cases rise to highest in 33 years
In 2000, measles was declared eliminated from the U.S. Elimination is “public health jargon” that means the virus was no longer continuously spreading within the U.S. for a period of at least 12 months. Instead, the only cases were isolated occurrences when the virus was introduced through travel. Immunization rates were high enough that the virus could not find enough susceptible people to keep spreading.
Unfortunately, several areas with decreased immunization rates now exist, enabling the virus to continue to spread. We are currently entering our seventh month with continuous spread of measles in the U.S. If cases continue through January 2026, the U.S. will lose its status as a country that has eliminated measles.
More than the loss of a status, it is important to realize that this means people have continued to be infected with measles. Indeed, the number of people infected with measles is greater than any outbreak in the last 33 years. Further, three people in the U.S. have died from measles in 2025, which equals the total number of deaths from measles in the last 25 years combined. The two children who have died from measles this year are the first child deaths from measles in the U.S. since 2003.
Find out more: