Vaccines and side effects

Vaccines are medicines. Like all medicines vaccines can cause side effects, although not everyone gets them. The chance of getting side effects varies from person to person. 

Some side effects are more common than others. There is a scale used to describe how often side effects occur. The scale ranges from very common to very rare. Read more about medicines and side effects.

The most common side effects of vaccines are a slight fever and swelling and/or redness at the injection site. This is your immune system’s natural response to the vaccine. It is nothing to be alarmed about and usually doesn't last very long.

At the other end of the scale, about 1 in 1 million people may experience a severe allergic reaction called anaphylaxis. This is why you are asked to wait for 15–20 minutes after vaccination, so trained nurses can immediately treat the allergic reaction. Treatment is very fast and effective. 

Each vaccine may also have their own rare but particular reactions. Find out more about vaccines in New Zealand.

Before a vaccine is ever recommended for public use, it undergoes a lot of testing, both in labs and in volunteers. These tests answer important questions, such as the following:

• Is the vaccine safe?
• What dose (amount) works best?
• How does the immune system react to it?

In Aotearoa New Zealand, before vaccines are used in the community they must be approved by Medsafe, the New Zealand Medicines and Medical Devices Safety Authority. Medsafe only grants consent for a vaccine to be used once they are satisfied it’s safe and effective enough to use.

Once a vaccine is approved, health experts continue to monitor the vaccine’s safety and effectiveness. Suspected side effects to vaccines are reported to the Centre for Adverse Reactions Monitoring (CARM), based at the University of Otago in Dunedin. 

The purpose of monitoring is to watch for adverse events (possible side effects). Monitoring a vaccine after it is licensed helps ensure that possible risks associated with the vaccine are identified. Read more about the vaccine safety monitoring process(external link). 

The safety of vaccines is often a topic of media stories and blog postings, but the information is not always accurate. Knowing where to get credible and reliable information is important. In Aotearoa New Zealand, the trusted sources of information about vaccines include:

• Medsafe(external link)
• Health New Zealand | Te Whatu Ora(external link)
• Healthify He Puna Waiora, NZ
• your healthcare provider.

Knowing the facts can help you feel more confident about making sure you and your family/whānau have all your vaccinations. Read more about vaccines in New Zealand.

All ingredients of vaccines play a necessary role. This may be in triggering your body to develop immunity or in ensuring that the final product is safe and effective.

The following are some common ingredients:

• Adjuvants – these help boost your body’s response to the vaccine. Adjuvants are also found in antacids, buffered aspirin, antiperspirants, etc.
• Stabilisers – these help keep the vaccine effective after it has been manufactured. Stabilisers are also found in foods such as jelly. They also naturally occur in your body.
• Formaldehyde – this is used to prevent contamination by bacteria during the vaccine manufacturing process. It also resides in your body naturally, more so than in vaccines. Formaldehyde is also found in the environment, preservatives and household products.
• Thimerosal – this is also used during the manufacturing process, but is no longer an ingredient in any vaccine except multi-dose vials of the flu vaccine. Single dose vials of the flu vaccine are available as an alternative. No reputable scientific studies have found an association between thimerosal in vac­cines and autism.

There are no traces of peanut, tree nuts, wheat, soy seeds or seafood in vaccines. Chicken and egg products are used in the manufacture of the MMR vaccine and some flu vaccines, but they are safe to use if you have an egg allergy. The yellow fever vaccine is the only vaccine that is not safe in people with an egg allergy. 

When you get a vaccine, any side effects from the vaccine happen within days and weeks, not years. If you are going to experience a side effect following vaccination, most will occur within the first week, and up to six to eight weeks later. When you are given a vaccine, the vaccine itself is gone from the body within hours to days – what you’re left with is the immune response. 

Sometimes reports of new side effects can take months to emerge in the general population. However, this isn’t because the side effects themselves take a long time to occur after vaccination. It’s because some side effects are so uncommon that they can’t be detected in clinical trials, which typically involve thousands of people.

It is important to let your doctor, pharmacist or nurse know about your side effects to any vaccine that you or your child may have. Reporting side effects helps protect everyone’s health. Read more about reporting side effects.