One Shot and Done
April 7, 2021
In contrast to the Pfizer and Moderna vaccines, Johnson and Johnson’s Janssen COVID-19 vaccine is a viral vector vaccine, which uses a modified adenovirus to store and carry instructions. Adenovirus 26 is another virus that can enter cells but would not replicate itself or cause illness.
Because the vaccine uses another virus vector, the immune system will develop an immune response to both COVID-19 and the adenovirus, making a second dose unnecessary.
“One of the benefits is that it only requires one dose. But because it is only one dose, it’s not as protective as the [Pfizer and Moderna vaccines],” Wang said. “The second dose won’t do anything because you already have the immune response from your first dose for another virus vector as well.”
Therefore, the vaccine trials did not include a second shot.
“When you inject the second shot, the immune response elicited by the first shot will limit the effectiveness of your second shot,” Jin said. “It’s also based on their data; one shot has already provided enough protection, and that’s how the clinical trial was set up.”
Reports show that Johnson and Johnson’s vaccine has a 75% efficacy rate. It also takes extra time to develop and grow the modified virus used in the vaccine compared to using mRNA.
On the other hand, since the adenovirus contains double-stranded DNA and has a protective coat surrounding the genetic material, the Johnson and Johnson vaccine is less fragile than the mRNA vaccine, making storage much easier. The vaccine must be kept in a refrigerator between 2 and 8 degrees Celsius; this simpler storage method allows it to be accessible to more people worldwide.