A new Journal of Toxicology and Environmental Health study reports that the higher the proportion of infants and toddlers receiving recommended vaccines, the higher the state's rate of children diagnosed with autism or speech-language problems just a few years later. This analysis is sure to rekindle the debate about vaccine safety.

For that conversation to produce useful results, we must start by defining terms. A "dose of vaccine" refers to each vaccine or antigen given to increase immunity against one specific disease. For chicken pox, a child receives one dose of vaccine through one shot.

By contrast, an "immunization event" refers to each separate administration of a vaccine or bundle of vaccines — through a shot, orally, or nasally. The MMR shot for mumps, measles and rubella involves three doses of vaccine but is one immunization event.

The critical number is how many doses of vaccine a child receives. Why? If a vaccine is strong enough to confer immunity against a disease, it is important enough to count separately.

Clear definitions, analysis of CDC's "General Recommendations on Immunization," and confirmation by Dr. Andrew Kroger, lead author of the definitive report on these recommendations, produce the answer to the not-so-simple-after-all question posed above.

In all, the federal government recommends 36 doses of vaccine, addressing 14 different diseases, for every U.S. child under age 2. An on-schedule child will receive a dose of vaccine for hepatitis B at birth, eight doses of various vaccines at 2 months, seven additional doses at 4 months, and four to seven more doses at 6 months.

Infants and toddlers receive these vaccine doses through 26 separate immunization events — mostly shots. If a child misses vaccinations because of illness or scheduling problems, following CDC's catch-up schedule usually results in extra doses at a later date.

The federally recommended doses of vaccine for every child during the first two years of life are: three doses each for hepatitis B, polio, flu, and HIB (12 doses in all); two doses each for hepatitis A and rotavirus; four doses for pneumococcal infections; one dose for chicken pox; three doses through the combination MMR vaccine for measles, mumps and rubella; and 12 doses through four separate administrations of the combination DTaP vaccine for diphtheria, tetanus and pertussis (whooping cough).

Some infants and toddlers receive still more doses of vaccine — if they switch to pediatricians who use different "combined" vaccines, if they are at high risk for certain diseases, if lost or incomplete records lead to duplicate immunizations, and depending on the time of year they were born (for flu shots) or the brand of vaccine used.

While testing is routine for individual vaccines as they are licensed, research on the both short- and long-term effects of multiple doses of vaccine administered to very young children during the critical birth-to-2 developmental window is sparse to nonexistent.

In addition to the number of doses, vaccine ingredients can be problematic, especially for susceptible subgroups. First are adjuvants, substances added to boost effectiveness and allow smaller doses of vaccine antigen to be used. The most common adjuvant is aluminum, which is found in vaccines for hepatitis and diphtheria-pertussis-tetanus.

Second are preservatives — such as thimerosal, which is 49.6 percent mercury. Thimerosal is still contained in many flu shots, although it was, except for trace amounts, removed from other child vaccines a decade ago. Many child vaccines (including those for diphtheria-pertussis-tetanus, HIB, and hepatitis) contain formaldehyde, which was just added to the government's list of known human carcinogens.

Third are ingredients to which some people have severe allergies: stabilizers such as gelatin, and eggs or other proteins that are used to prepare vaccines for flu, MMR, and other immunizations.

The ongoing debate about vaccines and their safety needs to incorporate these basic facts as our country seeks to answer the critical Goldilocks question: Too many? Too few? Or just right?

Margaret Dunkle is senior research scientist at the Department of Health Policy at George Washington University and director of the Early Identification and Intervention Collaborative for Los Angeles County. She also has a family member who is vaccine-injured. Her email is [email protected].