Frequently asked questions about vaccine schedules

Many people have questions about vaccine schedules. This page answers some of the most frequently asked questions about vaccine schedules.

How are vaccine schedules made?

Before a vaccine is added to the routine schedule, it must first be approved for use by Health Canada.  Once a vaccine has been approved for use, recommendations about how to use the vaccine are then made by Canada’s National Advisory Committee on Immunization (NACI). This committee is made up of experts in the fields of pediatrics, infectious diseases, immunology, medical microbiology and public health. NACI makes recommendations for who should receive certain vaccines based on the burden of illness in Canada, as well as evidence about the vaccine’s safety and effectiveness. Their recommendations are published here.

The ministries of health in each of the provinces and territories then use these recommendations from NACI to develop their vaccine schedules and recommendations, taking into consideration other health priorities.

Vaccines are only added to the vaccine schedule if they are known to be safe and effective. Even “new” vaccines have been studied for a long time to be sure that they are safe for your child and will provide important protection against disease.

Why do vaccine schedules change?

Sometimes vaccine schedules change. This can happen for a number of reasons, such as the development of new or improved vaccines, a new risk for disease, or new research on an existing vaccine.


  • The rotavirus vaccine is a fairly new vaccine and was added to the routine schedule in 2012 to protect children against a group of gastrointestinal viruses.
  • The hepatitis A vaccine was added to the routine schedule for all Aboriginal children in 2012 following several outbreaks of hepatitis A in Aboriginal communities.
  • The HPV vaccine schedule for Grade 6 students changed from three doses to two doses in 2014 after new research showed that two doses provided as much protection as three for this age group.

Why do children get vaccines starting at such a young age?

The recommended schedule is designed to protect infants and young children early in life, when they are most vulnerable, and before they are potentially exposed to vaccine-preventable diseases.

Infants and young children are at greater risk from vaccine-preventable diseases because their immune systems are less mature, and less able to fight off infection. If an infant or young child were to get a vaccine-preventable disease, it could be very serious and life-threatening.

Vaccinating children on time (starting at 2 months of age) gives them the best protection as early as possible.

Learn about why it's important to follow the recommended schedule and get all vaccines on time.

Why do most vaccines require more than one dose?

Most vaccines require more than one dose over time to produce immunity and long-lasting protection. The number of doses needed depends on the type of vaccine.

There are two main types of vaccines: live attenuated vaccines and inactivated/subunit vaccines. Learn what these terms mean below, and how they affect the number of doses you need for full protection.

Live attenuated vaccines

Live vaccines are made from a weakened version of the germ (virus or bacteria) that causes the disease. When a person receives a live vaccine, the germ in the vaccine multiplies (grows) in the body and stimulates an immune response almost identical to that produced by natural infection. Because it is weakened, the vaccine germ cannot multiply enough to cause disease in people with healthy immune systems. However, live vaccines are not given to people with very weak immune systems as they may develop the disease the vaccine is meant to protect against.

One dose of a live vaccine usually produces immunity and long-lasting protection because the vaccine stimulates an immune response that is almost identical to that produced by natural infection. But because not all individuals respond to the first dose, a second dose is usually recommended. The measles, mumps, rubella (MMR) vaccine and chickenpox (varicella) vaccine are examples of live attenuated vaccines.

Inactivated/subunit vaccines

Inactivated vaccines are made from a killed germ (or parts of the germ for subunit vaccines) that causes the disease. Since the vaccine does not contain a live germ, there is no growth of the germ in the vaccinated person, and this leads to a weaker immune response compared to live vaccines. Due to the weaker immune response, inactivated/subunit vaccines almost always require more than one dose. In general, the first dose “primes” the immune system and the second and/or third dose produces a protective immune response. These vaccines are safe even for those with weakened immune systems, because the killed germ cannot cause disease.

Immunity from these vaccines can decline with time and booster doses are often needed to ensure continued protection. The tetanus, hepatitis B and pertussis vaccines are examples of inactivated/subunit vaccines.

Influenza vaccines

Unlike other vaccines, a dose of the influenza (flu) vaccine is needed every year. That’s because every year, there are different influenza viruses that people can catch. As a result, a new version of the vaccine is created every year to provide best protection against the current influenza viruses. Most people need one yearly dose of influenza vaccine, but children under 9 years of age who have never had an influenza vaccine need two doses.

There are live and inactivated versions of the influenza vaccine. The inactivated version (given as an injection) is safe for people with weakened immune systems.

Is it safe for children to get more than one vaccine at a time?

Yes. Before vaccines are given together, studies must show that the vaccines are just as safe and effective when given together as they are when given separately. Research shows that vaccine side effects, such as temporary pain or swelling at the injection site, are similar when vaccines are given together as when given separately.

Getting more than one vaccine at a time means your child is protected as quickly as possible during the time when they are most vulnerable, and it also means fewer clinic visits.

Will getting multiple vaccines overwhelm my baby’s immune system?

Your baby’s immune system is amazing. From the day your baby is born, their immune system is constantly working to protect them against the thousands of germs (viruses and bacteria) that they are exposed to as part of daily life. Vaccines represent a much smaller challenge than this daily exposure, and even a combination vaccine is easy for your baby’s immune system to handle. In fact, scientists estimate that a baby’s immune system could theoretically handle thousands of vaccines even if they were given at the same time.

What are combination vaccines and are they safe?

Combination vaccines take two or more vaccines that could be given individually and put them into a single injection, so that children can get protection against several diseases with just one shot. The diphtheria, tetanus, pertussis, hepatitis B, polio, haemophilus influenzae type b (DTaP-HB-IPV-Hib) vaccine and measles, mumps, rubella (MMR) vaccine are examples of combination vaccines.

Combination vaccines are safe. They provide all the benefits of the individual vaccines but in a single injection, so your child does not need to receive as many shots. Before a combination vaccine is approved for use, studies must show that the combination vaccine is just as safe and effective as each of the individual vaccines given separately. Research shows that the side effects of combination vaccines are similar to those of the individual vaccines given separately. The vast majority of side effects are very mild, such as temporary pain and swelling at the injection site.

If my child missed getting a vaccine do they need to start the series over again?

No. If your child missed a dose of vaccine, they do not need to start the whole series over again. Your child can pick up where they left off, regardless of the time that has passed between doses. However, it is best to follow the recommended schedule as closely as possible so that your child is completely protected sooner.

I can’t remember if my child got a vaccine and have no record. Is it safe to repeat vaccines?

Yes. Getting an extra dose of vaccine when an immunization history is unknown is not harmful. It is better to ensure protection by getting the vaccine than risk leaving your child unprotected.

You may be able to locate your child’s record if you have lost it. Click here for tips on locating immunization records.

I didn’t immunize my child but have changed my mind. Is it too late to catch up?

For most vaccines, it’s never too late to catch up. There is a catch-up schedule that health care providers use to ensure children are protected as soon as possible.