(Updated as of January, 2009) CPCHE made the recommendation about avoiding insertion or replacement of mercury amalgam fillings during pregnancy based on studies showing that measurements of mercury in the body correlate with the number of dental fillings, and that the largest exposure appears to occur during placement or replacement of fillings. While there have been specific methods developed  by dentists to reduce exposure during these procedures, given the high vulnerability of fetuses to mercury (as demonstrated from other studies), Health Canada, and CPCHE partners, deemed it wise to make the most cautious recommendation of avoidance.

We also focused our recommendations about mercury on the issue of methylmercury in fish because of a large body of research indicating that fish consumption can be a significant source of mercury exposure for humans. However, fish are also an important source of nutrients, particularly of omega-three fatty acids. We were, and are, concerned with ensuring that the public is aware of fish advisories and can make informed decisions when choosing to eat fish. However, we were unsure whether or not thimerosal in the influenza vaccine should be a concern, and so made no specific recommendation in that regard.

Ethylmercury, the form of mercury found in thimerosal, is different from that found in fish (methylmercury) and from the elemental mercury used to make amalgam fillings.  Ethylmercury appears to be eliminated much more rapidly from the body than methylmercury.  The Public Health Agency of Canada's National Advisory Committee on Immunization (NACI) stated in 2005: "There is no legitimate safety reason to avoid the use of thimerosal-containing products for children or older individuals, including pregnant women."

In July of 2007, NACI reaffirmed this view in an updated statement on thimerosal. The NACI statement summarizes a review of the literature and concludes that “the weight of evidence now available refutes any link between thimerosal and autism.” In addition, the NACI issued a Statement on Influenza Vaccination for the 2007-2008 Season. In this updated statement, the NACI provides lists of at-risk people who should get a flu shot. NACI has expanded this list to include pregnant women because of their greater risk, compared to non-pregnant women, of respiratory complications, including hospitalization, should they get the flu. It further notes that pregnant women with additional health issues, such as asthma, are at even greater risk of respiratory complications. In an addendum to this statement published in October of 2007, NACI reiterates that "large health databases have demonstrated that there is no association between childhood vaccination with thimerosal-containing vaccines and neurodevelopmental outcomes, including autistic-spectrum disorder. They further note that, "nevertheless, in response to public concern, influenza vaccine manufacturers in Canada are currently working towards production and marketing of thimerosal-free influenza vaccines."

While the NACI conclusions about autism are reassuring, it is also true that the precautionary step was taken in the late 1990s to stop using thimerosal in childhood vaccines. Moreover, the 2007 updated NACI statement on thimerosal supports the ongoing pursuit of alternatives to thimerosal as a vaccine preservative. Thimerosal has not been used in any routine childhood vaccines in Canada since about 1999.  It remains only in multi-dose vials of influenza and Hepatitis B vaccines to keep the vaccines free from contamination after the multi-dose vials are opened.  Until manufacturers can fully remove thimerosal in all flu vaccines, single doses of influenza vaccine without preservatives are available, depending upon availability from suppliers, although special orders are required and two injections are needed, increasing the cost and inconvenience. Pregnant women can discuss this option with their physicians.

While the available evidence for links between certain substances and harm to health almost always leaves some uncertainty, the weight of current evidence indicates it is unlikely that the small amount of ethylmercury (thimerosal) present in the flu vaccine will pose a risk to an unborn child. At the same time, the NACI statement provides clear evidence of greater risk for pregnant women of serious respiratory problems if they get the flu. Should a pregnant woman still feel concerned, she could, as a precaution, ask her doctor for the single dose vaccine that does not contain thimerosal.