Vaccines and immunization: Vaccine safety

Vaccination is one of the best ways to prevent diseases. Childhood vaccines save 3.5 to 5 million lives every year. In 2021, COVID-19 vaccines are estimated to have saved 14.4 million lives globally.

Together with governments, vaccine manufacturers, scientists and medical experts, WHO's vaccine safety programme is constantly helping monitor the safety of vaccines.

This helps ensure that vaccines are safe for you and your family.

Most childhood vaccines have been in use for decades, with millions of people receiving them safely every year, routinely or in response to specific disease threats.

For example, more than 13 billion doses of COVID-19 vaccines have been safely administered globally since 2021, preventing millions of cases of severe disease and deaths.

Before any vaccine is introduced in a country, the vaccine developed in the laboratory undergoes rigorous and stringent testing through multiple phases of clinical trials. Health authorities carefully evaluate the results of these trials to help ensure that the vaccine meets the highest safety and efficacy standards before being considered suitable for use.

Once vaccines are introduced and used in countries, national health authorities continuously monitor vaccine safety to detect and immediately respond to potential concerns. In case of an adverse event, an independent group of experts assesses whether such an event is related to vaccines.

WHO assists countries in strengthening vaccine safety monitoring and response systems. At the global level, the Global Advisory Committee on Vaccine Safety (GACVS),  an independent group of experts, evaluates reports on vaccine safety and provides authoritative and scientific advice to WHO.

Vaccines are very safe. Like any medicine, vaccines can cause side effects. However, these are usually very minor and in short duration, such as a sore arm or a mild fever. More serious side effects are possible but extremely rare.

A person is far more likely to be seriously harmed by a disease than by a vaccine. Vaccine-preventable diseases like measles, meningitis, hepatitis B or polio can be deadly or cause severe illness and disability. Possible complications of these diseases can include blindness, deafness, paralysis, brain damage, infertility, cancer, birth defects and death.

Yes, it is safe to receive several different vaccines during one visit. Combination vaccines are as safe and effective as individual vaccines.

Receiving several vaccines or combination vaccines in one visit is important to protect children from various diseases as early as possible. This also makes it easier to complete the recommended doses on time.

Receiving multiple doses also does not overwhelm the immune system. The antigens present in vaccines are a small fraction compared to what our bodies naturally encounter every day.

Once a promising vaccine candidate is identified, it will first undergo thorough laboratory testing. This includes careful examination and testing of the vaccine candidate and its supporting ingredients. These tests evaluate the safety of the vaccine and how well it prevents a disease.

If positive results are achieved in the laboratory, a manufacturer can then apply to do clinical trials. These trials typically involve following up with several thousands of vaccinated healthy volunteers. The trials are also carefully monitored by national regulatory authorities.

The trials are bound by strict regulations and take place across three main phases:

• During Phase I, small groups (for instance, approximately 20-50 people) receive the vaccine. This phase will assess the safety, side effects, appropriate dosage, method of administration and composition of the vaccine.
• If it is successful, it will proceed to Phase II. At this stage, the vaccine is usually given to several hundred people. This group will have the same characteristics (e.g., age, sex) as the people for whom the vaccine is intended to be given.
• In Phase III, the vaccine is usually given to thousands of people to help ensure it is safe and effective for broader use.

The results of all these studies will be assessed when regulators decide whether to approve a vaccine.

Additional studies may also take place after a vaccine is introduced. They enable scientists to monitor efficacy and safety among an even larger number of people over a longer timeframe.

In countries where vaccines are manufactured, national or regional regulators oversee the vaccine’s development. This includes approving clinical trials, evaluating their results, and making decisions on licensing. All decisions must adhere to strict international standards on ethical clinical practice.

The national regulatory authority (NRA) of each country decides if a vaccine will be introduced in the country’s immunization program. To assist NRAs, WHO offers a process called prequalification. Prequalification facilitates informed decision-making on the approval and procurement of vaccines, especially in countries with limited resources for conducting extensive evaluations.

In prequalification, WHO independently assesses evidence on the quality, safety, and efficacy of vaccines and other medical products, helping ensure that only those that meet stringent international standards are used in public health programs. 

As with all medicines, the safety of a vaccine must be continuously monitored, even after trials and vaccine introduction.

Monitoring considers reports from several sources. Nationally, these include the people who get vaccines, parents or caregivers, and health workers. These reports are then submitted to national health authorities.

At a regional and global level, WHO and UNICEF support countries in collecting and monitoring this information and ensuring that countries have the most up-to-date evidence on available vaccines.

If a potential problem is identified following vaccination, a thorough investigation will take place. Investigations involve a detailed examination of the case, including medical assessment. If necessary, studies are conducted.

During these investigations, it is extremely rare that health problems are found to be caused by the vaccine itself. Health events are most often found to be coincidental, i.e., entirely unrelated to vaccination.

Sometimes, events are related to how the vaccine has been stored, transported or administered (for instance, errors are made in preparing the vaccine). Such errors can be prevented by training health workers and strengthening vaccination programmes.

In the rare cases where a genuine adverse reaction is suspected, the vaccine may be suspended from use. Further investigations will take place to determine what exactly caused the event. Programmes will then need to put corrective measures in place.

WHO works to ensure that everyone, everywhere, is protected by safe and effective vaccines.

To do this, WHO helps countries set up rigorous safety systems for vaccines – alongside other medicines - and work to develop the strictest standards for their regulation.

Together with external scientists, WHO experts continuously monitor and update the safety profile of vaccines. With external partners, it also helps countries investigate and communicate if potential issues of concern arise.

Events that are reported to WHO are evaluated by an independent group of experts (the Global Advisory Committee on Vaccine Safety, or GACVS). On behalf of this committee, WHO issues regular statements on vaccine safety.

For more information on how vaccines work, see our Q&A on vaccines.