If someone receives the vaccine, can they still be a carrier?


March 16, 2021: Recently Pfizer announced that, two weeks after people receive their second dose, the vaccine appears to be 94% effective at preventing asymptomatic infections. According to data, asymptomatic spreaders are likely the biggest causes of COVID-19 transmission, so vaccines that prevent a lot of asymptomatic infections will likely lead to big decreases in transmission.

Early data on whether the COVID-19 vaccines are able to reduce transmission look positive, but more research is needed to get to a conclusion. A person who is vaccinated for COVID-19 may still be able to transmit the virus.

This article was published on
March 18, 2021

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What our experts say

According to the FDA, many vaccines that protect against viral illnesses might also reduce transmission. In a February 2021 pre-print article, scientists looked at transmission among people who received the Pfizer COVID-19 vaccine and then subsequently became infected with the virus. The scientists found viral loads — which can indicate contagiousness — to be lower in patients who became infected 12-28 after their first shot. This may hint that the vaccine could lower infectiousness and impact the spread of COVID-19. Trials of the Moderna vaccine and the Oxford-AstraZeneca vaccines suggested it might be possible that the vaccines prevent people from being infected entirely (rather than just reducing the severity of symptoms), but more validation is needed. The exact amount of reduction in transmission as a result of vaccination is still unknown. It is also unclear how long the protection lasts. Additionally, the present vaccines are reported to have about 95% efficacy. This means that about 5% of people might not be protected, despite being vaccinated. The U.S. C.D.C. and public health professionals advise wearing a mask (though the group now recommends wearing a cloth mask over a surgical mask when not wearing disposable masks or KN95/N95 masks) and maintaining physical distance even after getting vaccinated, in order to protect those that are not vaccinated as yet and to stop further spread in the community.

Context and background

The flu vaccine is another example where there is less chance for people who are vaccinated to get the flu or fall severely ill, but they are capable of spreading influenza around in the community


  1. Can COVID vaccines stop transmission? Scientists race to find answers (Nature)
  2. Decreased SARS-CoV-2 viral load following vaccination (medRxiv)
  3. Can you still transmit Covid-19 after vaccination? (BBC)
  4. Oxford vaccine could substantially cut spread (BBC)
  5. Pfizer’s vaccine appears to reduce coronavirus transmission (ScienceNews)
  6. COVID-19 Vaccine: What You Need to Know (Johns Hopkins)
  7. Facts about COVID-19 Vaccines (US C.D.C.)
  8. Here’s Why Vaccinated People Still Need to Wear a Mask (NYT)
  9. Could people still be infectious after they're vaccinated for COVID-19? (CBS News)
  10. If a person is vaccinated against COVID-19, will they be able to spread the virus to susceptible people? (Children's Hospital of Philadelphia)
  11. Once someone is vaccinated, do they still have to wear a mask? Your COVID-19 vaccine questions answered (CBC)
  12. Maximizing Fit for Cloth and Medical Procedure Masks to Improve Performance and Reduce SARS-CoV-2 Transmission and Exposure, 2021 (U.S. CDC)
  13. Pfizer-BioNTech COVID-19 Vaccine Frequently Asked Questions (FDA)
  14. Episode #23 - I am vaccinated, what next? (WHO)
  15. Key Things to Know About COVID-19 Vaccines (U.S. CDC)
  16. Real-World Evidence Confirms High Effectiveness of Pfizer-BioNTech COVID-19 Vaccine and Profound Public Health Impact of Vaccination One Year After Pandemic Declared (Pfizer)

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