The FDA has authorized for emergency use vaccines created by Pfizer/BioNTech, Moderna, and Johnson & Johnson. The Pfizer and Moderna vaccines use what’s called messenger RNA (mRNA), which in this case is a synthetic material that carries the genetic code used to make the SARS-CoV-2 spike protein. The spike is the part of the virus that attaches to human cells. The mRNA is showing your immune system a part of the virus, so that when your body encounters the whole virus in real-life, it recognizes the invader and is able to mount a quick immune response, activating antibodies (B and T-cells) to fight it.
Vaccines using mRNA are new but the technology has been around for decades and has been used in treatments for other diseases such as cancer and cystic fibrosis. While many vaccines use a weakened or inactive version of the virus to create an immune response, these vaccines don’t so there is no risk of their causing disease.
The J&J vaccine is an adenoviral vector vaccine, which means it contains an engineered virus that can't cause disease. Within that virus is the genetic code for the SARS-CoV-2 spike protein. After the shot, the adenovirus enters the cells and then shows that spike protein to your immune system so that if you encounter the whole virus in real-life, your body can mount a quick and effective response to fight it.
On April 13, distribution of the J&J vaccine for COVID-19 was paused by the FDA while it conducted an investigation into reports of rare blood clots in a few patients shortly after receiving that vaccine. On April 23, J&J vaccinations resumed following an extensive safety review by the CDC and FDA, which determined the benefits of the single-shot J&J vaccine outweigh the exceptionally rare risk of blood clots. An independent panel of experts reiterated the vaccine meets the FDA’s standards for safety and effectiveness while also making sure that more information about the possible side effect is available to the public.