An issue of confusion has been whether the COVID-19 vaccines from Moderna or Pfizer or Astrazeneca and Oxford University can cause someone to become infected with the coronavirus that causes COVID-19. The answer is NO!
These vaccines only let the vaccinated person make one protein from the coronavirus. One protein is not enough to make an entire virus. So, there is no way that these vaccines can cause anyone to become infected with SARS-CoV2 or any other virus.
One protein from the virus can trigger an immune response and this is what these vaccines do. They let the vaccinated person make this one viral protein called Spike (S) so that the immune system can recognize it and develop a memory of this protein.
This immune memory is in the form of B cells that make antibodies and T cells that recognize peptides (break down products) of the S protein. So with this immune memory, the vaccinated person can fight the infection and not get seriously ill if infected by the SARS-CoV2 virus that causes COVID-19.
However, this vaccinated person can still get infected and spread the virus to someone else. The vaccinated person is protected from getting the severe form of COVID-19 that causes people to go into severe respiratory distress or even die. Even if you have been vaccinated against COVID-19, keep wearing a mask! [Update 7 April 2021: Research shows that the vaccines reduce the chance of transmitting the virus if you are asymptomatic. See Vaccine Effectiveness Against Severe COVID-19 and Transmission]
Also of interest
Vaccine Effectiveness Against Severe COVID-19 and Transmission
Cite as: N. R. Gough, COVID-19 Vaccines Do Not Cause COVID Infection. BioSerendipity (18 January 2021).