This article is part of a series of explainers on vaccine development and distribution. Learn more about vaccines – from how they work and how they’re made to ensuring safety and equitable access – in WHO’s Vaccines Explained series.
How vaccines are tested and monitored
Most vaccines have been in use for decades, with millions of people receiving them safely every year, routinely or in response to specific disease threats.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. Globally, reports of vaccine safety events are collected and analyzed in a database at the Uppsala Monitoring Centre, a WHO collaborating centre that specializes in monitoring the safety of medicines and vaccines. This global system helps detect even weak and rare safety signals. In addition, 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. On behalf of this committee, WHO issues regular statements on vaccine safety.
Getting vaccinated is safer than getting infected
Vaccines train your immune system to recognize the targeted germs virus and create antibodies to fight off the disease without getting the disease itself. After vaccination, the body is ready to fight those germs more effectively, preventing illness, complications and even death –— risks that come with natural infection. Billions of people have been safely vaccinated against COVID-19. All of the approved COVID-19 vaccines have been carefully tested and continue to be monitored.
Getting vaccinated on time, according to the schedule recommended by your health authorities, ensures you stay as safe and protected as possible. Delaying vaccination leaves you at risk. If you get infected, your body may not have enough time to build immunity, unlike those already vaccinated.
For example, missing the measles vaccine puts children at high risk of severe complications, including pneumonia, brain swelling and even death. Measles spreads easily and can quickly affect unvaccinated individuals, especially children.
If you or your child missed any recommended vaccines, talk to a health-care worker about catching up.
What to expect during vaccination
Medical professionals can best advise individuals on whether or not, and when, they should receive a vaccine. Some individuals may need to delay or avoid certain vaccines due to their age, health conditions or other specific factors. If you have any concerns, discuss them with your health-care worker to ensure you get the best protection for your health.
A health worker will administer the vaccine, and the person receiving it will be asked to wait for 15–30 minutes before leaving the vaccination site. This is so that health workers can observe individuals for any unexpected reactions following vaccination.
Why it’s normal to have mild side effects from vaccines
Vaccines are designed to give you immunity without the dangers of getting the disease. It’s common to experience some mild-to-moderate side effects when receiving vaccinations. This is because your immune system is instructing your body to react in certain ways: it increases blood flow so more immune cells can circulate, and it raises your body temperature in order to kill the virus.
Mild-to-moderate side effects, like a low-grade fever or muscle aches, are normal and not a cause for alarm: they are signs that the body’s immune system is responding to the vaccine, specifically the antigen (a substance that triggers an immune response), and is gearing up to fight the harmful germ. These side effects usually go away on their own after a few days.
Common and mild or moderate side effects are a good thing: they show us that the vaccine is working. Experiencing no side effects doesn’t mean the vaccine is ineffective. It means everybody responds differently.
See WHO’s Vaccines Safety questions and answers to learn more about common side effects and find out who should consult with a doctor before vaccination.
Less common side effects
Individuals should alert their local health providers following vaccination if they experience any unexpected side effects or other health events – such as side effects lasting more than three days. Less common side effects include severe allergic reactions such as anaphylaxis; however, this reaction is extremely rare.
National authorities and international bodies, including WHO, are closely monitoring for any unexpected side effects following vaccination.
Receiving different vaccines during one visit
It is safe to receive multiple doses of different vaccines during one visit. Combination vaccines – many antigen types in one dose – are also 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