Vaccine makers must follow very strict scientific and health authority processes to bring a new vaccine to the public, even during a pandemic.

COVID-19 vaccines work in 3 ways:

• They lower the chance of getting the virus. This is called vaccine efficacy (how well the vaccine works).
• They can protect a person from getting seriously ill if they do get the virus. Though no vaccine is 100% effective, getting vaccinated lowers the chance of hospitalization and death from COVID-19.
• They make a vaccinated person less likely to pass the disease to others.

Phase 3 clinical trials (the last and largest phase of vaccine testing before authorization or approval) test efficacy, how effective a vaccine is at preventing COVID-19 disease and are conducted with hundreds of thousands of volunteers around the world. During Phase 3, some volunteers get a COVID-19 vaccine, and some get a “placebo”. A placebo is a harmless injection with no active drug or treatment, for example, like normal saline.

To learn how effective a vaccine is at preventing a specific disease, researchers assess the vaccine efficacy rate which compares:

• The number of volunteers who got COVID-19 after getting the vaccine being studied.
• The number of volunteers who got COVID-19 after getting a placebo.

Hundreds of thousands of people have volunteered to take part in different COVID-19 Phase 3 trials around the world. Some of these trials reported vaccine efficacy rates of over 90%. All of the authorized COVID-19 vaccines go beyond the FDA’s minimum efficacy level of 50%. In other words, the FDA would not authorize a vaccine unless it prevented COVID-19 or lessened its severity in at least 5 out of 10 (50%) of people who got the vaccine. After a vaccine is authorized, health authorities continue to monitor its safety and effectiveness.

The 2nd way COVID-19 vaccines work is to make it less likely for someone to get seriously ill, be hospitalized, or die, if they do get the virus. Though no vaccine is 100% effective, a vaccine is still the best way to protect against the virus. In countries where COVID-19 vaccines are being given, the vaccines appear to have greatly lowered the number of hospitalizations, intensive care unit (ICU) admissions, and deaths. By doing this, vaccines also help lower the overall burden on our healthcare systems.

Finally, the 3rd way the vaccines work is by lowering the spread of COVID-19 in communities. Evidence suggests that if a vaccinated person does get the virus, they are much less likely to pass it to others compared to an unvaccinated person. Lowering the spread in communities helps protect those who can't get vaccinated because of their age or underlying health problems. It also helps lower the chance of the virus mutating, and can stop new COVID-19 variants (slightly changed forms) from developing and spreading.

After receiving a vaccine, your body’s immune system needs a little time to learn how to detect and fight the virus. This maturation of the immune system can vary but is usually around a few weeks. Each authorized or approved COVID-19 vaccine has different speeds for how quickly it protects you. The speed of response can depend on factors such as the dosing schedules or the number of doses.

Based on clinical trial data of the currently authorized or approved COVID-19 vaccines, optimal protection is reached within a few weeks of vaccination. For two-dose vaccines, some early protection is achieved after the first dose.

The currently available data suggest that most non-vaccinated people who recover from COVID-19 infection develop an immune response that provides at least some protection against reinfection (getting it again) – this is called natural immunity. We’re still learning how strong this natural immunity may be, and how long it lasts. Both natural immunity and immunity from a vaccine are important aspects of COVID-19 that experts are trying to learn more about.

Researchers have found that the vaccines give a person high antibody levels. The COVID-19 vaccines also seem to do a good job of protecting against variants (slightly changed forms of the virus). It’s important to get all recommended doses of the vaccine to make sure you get the best protection possible.

Health authorities will continue to monitor the clinical trial volunteers for many months to check how long they are protected. The protection from the initial doses may lessen over time. And if so, booster doses may be needed to help strengthen your immune system. Booster doses will also help protect against new variants. As of November 2021, some countries have decided to recommend the use of booster doses of authorized vaccines, especially for higher risk populations such as the over 65s, frontline workers, the immunocompromised or adults with underlying medical conditions. If you have concerns, talk to your local healthcare provider, doctor or public health authority.

A booster dose is an extra dose you may receive after you have completed the initial primary vaccination schedule.

Research suggests that booster doses help to enhance and prolong immunity against a broader range of COVID-19 variants and hence reduce the risk of severe illness or hospitalization due to COVID-19.

Some countries are recommending booster doses especially for those with a weakened immune system. If you recently have had or suspect you have had COVID-19, you may consider delaying your booster dose.

If you have concerns, talk to your local healthcare provider, doctor or public health authority.

Maybe. None of the COVID-19 vaccines tested so far have been 100% effective at preventing COVID-19. So, some people who get the vaccine may still get COVID-19. However, clinical trials have shown that COVID-19 vaccines significantly lower the chance a vaccinated person will have severe symptoms, including hospitalizations and death.

There is also evidence to suggest that if a vaccinated person does get the virus, they are much less likely to pass it to others compared to an unvaccinated person. By lowering the chances of transmitting the virus, vaccinated people not only protect themselves, but also people around them. Lowering the spread in communities helps protect those who can't get vaccinated because of certain allergies to vaccine ingredients or a weakened immune system. Having fewer people with the virus also helps lower the chance of the virus mutating and can stop new COVID-19 variants (slightly changed forms) from developing and spreading.

We are still learning how vaccines will affect the spread of COVID-19 which is why it remains important to continue washing hands, using social distancing, and wearing a mask even after you get a vaccine. This guidance may change over time and depending on government decisions in certain countries as we learn more.

During most COVID-19 vaccine clinical trials, people of different ages took part to confirm the vaccines safety profile and effectiveness in older age groups, who are most at risk of serious illness and death from COVID-19. Extremely frail older persons and persons above the age of 95 years were not included in clinical trials. Most of these trial results showed high levels of effectiveness in different ages groups, including in some cases those over age 65.

When deciding to get vaccinated or not, older adults should remember that they have been disproportionally affected by the COVID-19 pandemic:

• They have a higher chance of needing hospital care or dying if they get COVID-19, compared to younger adults.
• The chance of severe illness with COVID-19 goes up with age, with older adults at the highest risk.
• The greatest risk for severe illness from COVID-19 is among those aged 85 or older.

This is why the World Health Organization (WHO) has identified older adults as a priority group to get the vaccine. While vaccination is recommended for older persons due to the high risk of severe COVID-19 and death, very frail older persons with an anticipated life expectancy of less than 3 months should be individually assessed.

If you have concerns, talk to your doctor about whether or not you should get a COVID-19 vaccine.

According to the World Health Organization (WHO), people with underlying medical problems like high blood pressure, heart and lung problems, diabetes, obesity, or cancer, are at higher risk of having serious illness from COVID-19.

Clinical trials are designed to assess all races, genders, different ages, as well as people with well-controlled underlying health conditions, such as diabetes or heart disease. However, as with other vaccines, clinical trials for COVID-19 vaccines do not include people with a weakened immune system, such as people who are taking chemotherapy or bone marrow treatment. As COVID-19 vaccines are put into widespread use, safety and effectiveness data will continue to be collected, analysed, and reported for all populations, including people with conditions that suppress their immune response.

While COVID-19 vaccination might provide a lower level of protection in people who are immunocompromised compared with the rest of the population, it is still very important to get vaccinated, as well as continue washing hands, using social distancing, and wearing a mask event after getting a vaccine.

Some types of COVID-19 vaccines may be more suitable than others for people with weakened immune systems. If you have concerns, talk to your doctor about whether or not you should get a COVID-19 vaccine.

Researchers continue to collect and analyse data on new variants of the COVID-19 virus. The World Health Organization (WHO) is working with researchers, health officials, and scientists to understand how these variants affect the virus’s behaviour. They are also working to understand how, if at all, these variants impact how well vaccines work.

Viruses often change or mutate. A mutation is when the genetic material in the virus changes. When a virus is spreading widely in a community, it is more likely to mutate. That's why vaccinating as many people as possible to stop the spread is the best way to protect against new COVID-19 variants (slightly changed forms).

Mutations happen at different speeds in different viruses. New variants of a virus can spread more or less easily and can cause more or less disease than the original virus strain. Most viral mutations have little to no impact on the virus’s ability to cause infections and disease and they do not always affect how well a vaccine works against a virus.

The scientific community and health authorities are closely watching:

• How the SARS-CoV-2 (the virus that causes COVID-19) changes over time.
• How well COVID-19 vaccines can protect people against COVID-19 caused by any new variants of the virus that appear.

Given recent mutations with the SARS-CoV-2 virus, vaccine manufacturers continue to adapt and update their vaccines and booster doses.

Vaccines against some viruses keep working for many years after their development and provide long-lasting protection. These include vaccines against measles and rubella. Vaccines against other viruses like the flu need to be updated every year to keep working. This is because the flu virus mutates often and with large changes, with new versions appearing each flu season.

While we are learning more about COVID-19, we need to do everything possible to stop the spread of the virus and prevent mutations that may affect how well vaccines work over time. The more people who get vaccinated, the less likely it is that people will get sick from COVID19. This helps slow the spread of the virus and stop more dangerous variants from developing.