How many days after vaccination does protection against the coronavirus start?
Text updated on 2021-09-07
For effective protection against COVID-19vaccine, it is necessary to wait 14 days after the last dose of vaccine.
No matter which vaccine is used against COVID-19vaccine, the principle is the same: it is to teach our immune system to make specific antibodies against the coronavirus. These antibodies will be ready to neutralise the coronavirus when we encounter it later. The vaccine will also allow the development of "memory" immune cells, capable of immediately recognising the pathogen if it were to infect the individual at a later date. See the question What are the different types of vaccines against COVID-19 ?. But the action of the vaccine is not immediate: it takes several days or weeks for the immune system to mount an effective response against an unknown pathogen (virus, bacteria, parasite, etc.). For this reason, protection against COVID-19 is not effective on the day of the injection.
There are two scenarios to consider when determining when you are well protected against the disease COVID-19 after vaccination:
For people who have not had the COVID-19: You will get two doses of the vaccine (except for the Janssen vaccine which requires only one dose). After the first dose you have to wait 3-4 weeks to start being protected. You should therefore be careful to continue to apply barrier measures during the period following the first dose of vaccine, when you may believe - wrongly - that you are protected. The efficacy of RNA vaccines (Pfizer and Moderna) or the AstraZeneca vaccine against symptomatic forms of COVID-19 is about 60-70% 3 weeks after the first dose. 14 days after the second dose, the efficacy against symptomatic forms of COVID increases to about 70-90%, depending on the vaccine. For this reason, vaccination is considered complete and maximal 14 days after the second dose of vaccine.
For people who have already had COVID-19: You will only get one dose of vaccine. See the question I have already had COVID, should I be vaccinated? and protection against COVID-19 is maximal 14 days after the injection, especially against the variants.
Several studies are underway to find out how long immunity to the coronavirus SARS-CoV-2 lasts after vaccination.
Observational study of 324,033 people vaccinated with an RNA vaccine (Pfizer-BioNTech or Moderna) which showed that the vaccine was 60% effective against symptomatic forms of COVID-1920 days after the first injection and 91% 7 days after the second injection. The efficacy against severe forms of COVID-19 is 62% 20 days after the first injection and 98% 7 days after the second injection. The efficacy of RNA vaccines is also high against the Beta B.1.351 (detected in South Africa) and Gamma P.1 (detected in Brazil) variants which contain the E484 mutation.Chung, H., He, S., Nasreen, S., Sundaram, M., Buchan, S., Wilson, S., ... & Kwong, J. C. (2021). Effectiveness of BNT162b2 and mRNA-1273 COVID-19 vaccines against symptomatic SARS-CoV-2 infection and severe COVID-19 outcomes in Ontario, Canada.
Observational study of 503,875 people vaccinated with the Pfizer vaccine which showed that after one injection, the efficacy of the vaccine against symptomatic forms was 54.4%, 24 days after the injection of one dose.Chodick, G., Tene, L., Patalon, T., Gazit, S., Tov, A. B., Cohen, D., & Muhsen, K. (2021). Assessment of Effectiveness of 1 Dose of BNT162b2 Vaccine for SARS-CoV-2 Infection 13 to 24 Days After Immunization. JAMA Network Open, 4(6), e2115985-e2115985.
This study shows that a single dose of Pfizer/BioNTech or Astrazeneca vaccine provides 60-70% protection against symptomatic forms of COVID-1928 days after injection. For the Pfizer vaccine, protection against symptomatic forms of COVID-19 rises to 89% 14 days after the second dose. After the first injection of the vaccine (Pfizer or Astrazeneca), the efficacy against severe forms requiring hospitalisation is 80%. After the first injection of the Pfizer vaccine, the efficacy against fatal forms of COVID-19 is 85%.Bernal, J. L., Andrews, N., Gower, C., Stowe, J., Robertson, C., Tessier, E., ... & Ramsay, M. (2021). Early effectiveness of COVID-19 vaccination with BNT162b2 mRNA vaccine and ChAdOx1 adenovirus vector vaccine on symptomatic disease, hospitalisations and mortality in older adults in England. MedRxiv.
This study shows that in subjects who have had the COVID-19 and received a dose of RNA vaccine (Pfizer or Moderna), the immune response is present between 7 and 14 days after the first dose of vaccine.Saadat, S., Tehrani, Z. R., Logue, J., Newman, M., Frieman, M. B., Harris, A. D., & Sajadi, M. M. (2021). Binding and neutralization antibody titers after a single dose in health care workers previously infected with SARS-CoV-2. Jama, 325(14), 1467-1469.