Vaccine safety monitoring and what the evidence shows

How is vaccine safety monitored?

The CDC describes several complementary systems (overview). They answer different questions.

• VAERS (Vaccine Adverse Event Reporting System). Co-managed by the CDC and FDA, it is an early-warning system. “Anyone — patients, family members, healthcare providers and vaccine manufacturers — can submit a report,” and providers and manufacturers must report certain events. The CDC is explicit about its limits: it is a passive system, reports can be incomplete or contain errors, and “a report to VAERS does not mean that a vaccine caused an adverse event.” VAERS is used to spot patterns that other systems then investigate (CDC: VAERS).
• Vaccine Safety Datalink (VSD). A collaboration between the CDC and U.S. healthcare organizations, established in 1990, with 13 member sites. It uses electronic health records and checks weekly whether specific events occur more often after a vaccine than in a comparison group. It also informs the Advisory Committee on Immunization Practices (ACIP) (CDC: VSD).
• v-safe. A CDC system that sends check-ins by text or email asking people how they feel after vaccination. The CDC page currently describes it for COVID-19 and RSV vaccines (CDC: v-safe).
• Clinical Immunization Safety Assessment (CISA). A network of vaccine safety experts from the CDC, research centers and other partners, listed on the CDC's safety-systems page.
• Injury compensation. The National Vaccine Injury Compensation Program, run by HRSA, covers certain vaccines and injuries listed on a Vaccine Injury Table. Claims are filed as petitions with the U.S. Court of Federal Claims, and COVID-19 vaccine claims go to a separate program, the Countermeasures Injury Compensation Program. Eligibility rules and filing deadlines are on HRSA's pages (HRSA).

To report a possible reaction, talk to your child's practitioner and see the VAERS page above; anyone can file a report.

What do official sources say about aluminum?

Some vaccines contain aluminum salts as an adjuvant, an ingredient that helps the immune system respond. The CDC says aluminum salts “have been used safely in vaccines for more than 70 years,” that aluminum is also found in air, food and water, and that exposure from the recommended schedule “is low and is not readily absorbed by the body.” The same page notes that a September 2022 observational study using Vaccine Safety Datalink data “identified a possible association between exposure to aluminum from vaccines and later development of persistent asthma,” and that the CDC is not changing recommendations based on that single study and that more investigation is needed (CDC: Adjuvants and Vaccines).

Our vaccine and ingredient guide lists the aluminum amount on each product's label. The planner has an optional setting that limits aluminum-containing vaccines per visit. It is optional; the CDC page above describes exposure from the recommended schedule as low.

What do official sources say about thimerosal?

Thimerosal is a mercury-containing preservative. The FDA's page lists multi-dose presentations of three seasonal flu vaccines as the products that contain it as a preservative, notes that their single-dose versions are thimerosal-free, and states that all vaccines routinely recommended for children 6 and younger are available without thimerosal. The FDA also says a body of studies “support the safety of thimerosal-containing vaccines” and do not show evidence of harm (FDA: Thimerosal and Vaccines). The page was last updated in January 2025; check it for the current list.

What is known about spacing out vaccines?

There are two separate questions here, and the sources answer them differently.

• Is the recommended schedule safe? A 2013 National Academies (Institute of Medicine) review concluded there is “no evidence that the schedule is not safe.” It also said that few studies have comprehensively assessed the whole schedule, that no studies have compared health outcomes between unimmunized and fully immunized children, and that long-term studies of the cumulative number of vaccines have not been conducted. It recommended against randomized trials of alternative schedules and said the VSD should keep examining outcomes of people who choose alternative schedules (National Academies).
• Does giving vaccines at separate visits make them safer? None of the sources cited on this page shows that it does. The CDC's best-practice guidance says giving vaccines at the same visit has produced adverse-reaction rates “similar to those observed when the vaccines are administered separately,” with some exceptions (for example, a small increase in febrile seizures when the flu vaccine and PCV13 (an older pneumococcal vaccine) are given together in young children, about one extra case per 2,200 children). It also says that longer gaps between doses usually do not lower final antibody levels, but that “protection might not be attained until the recommended number of doses has been administered” (CDC: Timing and Spacing).

So the effect of spacing on safety has not been tested head-to-head, and the effect on timing of protection is clear: a child on a slower schedule is not fully protected until later. This site's planner follows the CDC's minimum ages and intervals so a slower plan stays valid, but it does not claim that a slower plan is safer. See delayed schedules, explained for the trade-offs.

Questions to bring to your practitioner

• Is there anything in our family history or my child's health that changes which vaccines or timing you recommend?
• Which safety concern is the main reason I am considering a different pace, and what do you know about it?
• If we space doses out, which diseases would my child be unprotected against, and for how long?

The pediatrician conversation guide has a printable list.