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  
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