Johnson & Johnson's single-dose coronavirus vaccine has become the third Covid-19 shot approved for use in the US. Previously, Pfizer and Moderna's two-dose vaccines had been approved.
None of the three vaccines contains additives that can sometimes cause strong reactions, such as antibiotics, preservatives or adjuvants, which are compounds used to boost the immune response that can add to the kick of any vaccine.
That means a very low risk of allergic reaction, especially life-threatening anaphylaxis. The US Centers for Disease Control and Prevention reports only a few cases of anaphylaxis in people who have been given the Moderna or Pfizer vaccines, and all were easily treated.
Only one case of anaphylaxis has been reported in the 44,000 people who have tested the Johnson & Johnson vaccine.
Plus, all three target a specific part of the spike protein called the receptor binding domain. As the name implies, it's the part the virus uses to grapple cells. Mutations to this particular region could weaken the efficacy of all three vaccines.
Luckily, all three appear to stimulate an overwhelming immune response — one much stronger than people get after a natural infection. So far, it appears to be enough to continue at least partially protecting people from the most concerning variants.