COVID-19 vaccines are safe and effective, saving numerous lives. Like all other vaccines, COVID-19 vaccines don’t fully protect everyone who gets vaccinated, and we don’t know how well they prevent spreading the virus to others. Therefore, in addition to vaccination, we must continue to take other measures to fight this pandemic.
Vaccine potency and effectiveness
All COVID -19 vaccines approved by the WHO for emergency use have passed randomized clinical trials testing quality, safety, and efficacy. Vaccines must be 50% or more potent to be approved. Following approval, it will be subject to ongoing safety and efficacy monitoring. But what is the difference between potency and effectiveness?
The efficacy of a vaccine is measured in controlled clinical trials, in which the number of people who get a “result of interest” (usually a disease) is compared to the number of people who get a placebo. After the study is complete, the number of sick people in each group is compared, and the relative risk of sickness is calculated based on whether the subjects were vaccinated or not. From this, we can calculate vaccine efficacy, which is how much the vaccine reduces the risk of disease. If the vaccine is highly potent, the number of people who get sick in the vaccinated group will be much lower than the number in the placebo group.
For example, if a vaccine has proven efficacy of 80%, that means that among those receiving the clinical trial, the risk of developing the disease is 80% lower in the vaccinated subjects than in the placebo-vaccinated subjects. This was calculated by comparing the number of cases in the vaccinated and placebo groups. 80% efficacy does not mean that 20% of the vaccinated group will get sick.
Vaccine effectiveness is a measure of how well a vaccine works in the real world. Clinical trials cover a wide range of people, covering a wide range of ages, including men and women, different ethnic groups, and people with known medical conditions, but clinical trials do not perfectly reflect the entire population. The efficacy observed in clinical trials applies to specific results obtained in clinical trials. Efficacy looks at how well the vaccine protects the entire community. Real-world efficacy may differ from efficacy measured in trials because we cannot accurately predict the efficacy of vaccination against a much larger and more diverse population in a real-world setting that is closer to reality.
Vaccine protection and timing
Vaccines offer strong protection, but it takes time to get that protection. People must receive all necessary doses of the vaccine to develop adequate immunity. For a two-dose vaccine, after the first dose, the vaccine provides only partial protection, and the second dose increases the degree of protection. The highest level of protection occurred in the weeks following the second dose. In the case of a single-dose vaccine, the greatest immunity to COVID-19 occurs several weeks after vaccination.
Vaccine Protection and Infection
Vaccines can prevent most people from getting sick with COVID-19, but they won’t keep everyone from getting sick.
Even if someone gets all the recommended doses of the vaccine and develops immunity a few weeks later, it is still possible to get infected. Vaccines do not provide adequate (100%) protection, so “breakthrough infections” can occur, where people who have been fully vaccinated can still contract the virus.
Symptoms may be milder if the vaccinated person becomes ill. Overall, vaccinated people rarely became seriously ill or died.
Vaccine protection and virus transmission
A COVID-19 vaccine is an important tool in responding to the pandemic and preventing severe illness and death. In terms of infection and transmission, vaccines provide at least some protection, but not as much protection against severe illness and death. More evidence is needed to determine how well the vaccine prevents infection and spread.
After vaccination, individuals should continue to take simple and easy precautions such as physical distancing, wearing a mask, keeping rooms well ventilated, avoiding crowds, cleaning hands, and covering coughs with flexed elbows or a tissue. If you are unwell, you should get tested even if you have been vaccinated. Please see advice on where to live and work. All these measures should be taken.
Vaccine protection and virus variants
When cases increase and transmission accelerates, new, more transmissible, dangerous variants are more likely to emerge, which are more transmissible or cause more severe disease.
From what we know so far, vaccines have proven effective against existing variants, especially in preventing severe illness, hospitalization, and death. But some variants had a slight effect on the vaccine’s ability to prevent mild illness and infections.
The vaccine triggers a broad immune response, so it may also be effective against the variant. This means that changes or mutations in the virus are unlikely to render the vaccine completely ineffective.
WHO continues to continuously review the evidence and update its guidelines as more evidence becomes available. For an update on what we know about variants of COVID-19, read our latest weekly outbreak update and our explanatory material on Variants of the Virus and Their Impact on a COVID-19 Vaccine.
The best way to protect against new variants is to continue with proven public health measures and roll out a vaccine. All COVID-19 vaccines approved by the WHO for emergency use have been fully tested and shown to provide a high degree of protection against severe illness and death. As stronger variants of the virus emerge, it’s important to get vaccinated when it’s your turn.