Most people think of vaccines this way: you get vaccinated, and your immune system prepares to fight a virus or bacteria that enters the body. The vaccine serves as the catalyst to set up protection, prevent infection, and prevent you from infecting others. Unfortunately, it is not quite that simple.
Not all vaccines work that way, and it’s not actually what the two COVID-19 vaccines authorized by the U.S. Food and Drug Administration—made by Moderna and Pfizer-BioNtech -— are designed to do.
Their effectiveness is measured by how well they protect people against moderate to severe COVID-19 disease—not how well they prevent infection or spread of the COVID-19 virus itself. Although the shots are 94% to 95% effective in preventing disease, there is no definitive data yet that proves they completely shut down the virus enough to stop it from moving from an infected person to someone else.
This past February, AstraZeneca and the University of Oxford reported the results of their late-stage, Phase 3 studies of their vaccine. The scientists said that their shot was 67% effective in protecting people from COVID-19 disease, and nearly 100% effective in shielding them from severe disease requiring hospitalization.
The scientists took weekly swabs from the study volunteers and tested them for the COVID-19 virus, and found 50% fewer positive tests among people who were vaccinated than among people who weren’t.
Public health officials say the findings still need to be confirmed. It is true that people who were vaccinated had less virus in them if they were infected, but one cannot conclude that they don’t spread the virus to others. As infectious disease experts like to say, all it takes is one virus to cause an infection.
It remains unclear whether getting vaccinated can stop you from spreading the virus if you’re infected: several months will need to pass in order to see if the virus moves from a vaccinated person to someone else.
That is why public health officials have said that even if you’re vaccinated, you need to continue wearing the mask, remaining six feet apart, and avoid large gatherings – even if your guests are all vaccinated. Even after getting the one- or two-shot vaccine, continue to mask, practice social distancing, and avoid crowded areas, especially indoors.