How Well the Vaccine Works. The J&J/Janssen COVID-19 Vaccine was 66.3% effective in clinical trials (efficacy) at preventing laboratory-confirmed COVID-19 infection in people who received the vaccine and had no evidence of being previously infected. People had the most protection 2 weeks after getting vaccinated.

How does the Johnson and Johnson COVID-19 vaccine work?

The Johnson & Johnson product is an adenovirus vaccine or a viral vector vaccine. Here is how it works.

The Johnson & Johnson vaccine delivers the virus' DNA to your cells to make the spike protein. An adenovirus acts as a delivery vehicle used to carry the coronavirus genetic material (DNA).

When do you have side effects from the Johnson & Johnson’s COVID-19 vaccine?

These side effects usually start within a day or two of getting the vaccine. Side effects might affect the ability to do daily activities, but they usually go away in a few days.