A McKesson Corporation employee packs a box of Johnson & Johnson Covid-19 vaccines for shipment to Shepherdsville, Kentucky March 1. Timothy D. Easley / Pool / Getty Images

The first Covid-19 single-dose vaccine was approved for use in the U.S. after advisers from the Centers for Disease Control and Prevention Unanimously decided to recommend the Johnson & Johnson shot for Americans 18 and older.

Distribution of the vaccine began on Sunday evening, immediately after CDC Director Dr. Rochelle Walensky had officially stamped the approval.

The Johnson & Johnson vaccine is a little different from the two already used in the US. Most importantly, it only takes one dose and is easier to use as it can be stored for up to three months at simple refrigerator temperatures. This makes it much easier to introduce compared to the Moderna and Pfizer vaccines.

The technology is different too. Moderna and Pfizer vaccines use a brand new technology called messenger RNA, or mRNA. They deliver genetic material directly into cells, which then follow genetic instructions to make tiny pieces that look like part of the coronavirus.

These tiny proteins stimulate an immune response, creating antibodies and immune cells that “remember” what they look like and are ready to respond quickly in the event of a new attack.

The Johnson & Johnson vaccine uses viral vector technology. A cold virus called adenovirus 26 is genetically engineered so that it can infect cells but does not replicate there. It cannot spread in the body and does not give people a cold. Like Pfizer and Moderna vaccines, it provides genetic instructions.

The Johnson & Johnson vaccine has shown 72% effectiveness in the US and provides 86% protection against severe forms of the disease in the country. The rate of effectiveness of Moderna and Pfizer in clinical trials ranged from 94% to 95%.

A version of this story appeared in the March 1 issue of CNN’s Coronavirus: Fact vs. Fiction newsletter. Sign up here to get the top headlines every day of the week.

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