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A health worker prepares a vaccine injection for administration. Image credit: UNICEF/Nahom Tesfaye

A third effective weapon was added to America’s arsenal against the coronavirus Saturday when the Food and Drug Administration granted emergency use authorization for a vaccine developed by Johnson & Johnson. The company said it would start shipping millions of doses early this week and would provide the United States with 100 million doses by the end of June. Together with 600 million doses of the nation’s first two authorized vaccines, made by Pfizer-BioNTech and Moderna, that are due to be delivered over the next four months, that ought to be enough to cover every American adult who wants to be vaccinated.

The new vaccine differs markedly from the two already in use in the United States. Here is how they compare.

One shot instead of two

The Johnson & Johnson vaccine is administered in a single shot, while the Pfizer-BioNTech and Moderna vaccines are given in two shots several weeks apart.

Vial of the Johnson & Johnson COVID-19 vaccine. Image: Johnson & Johnson

How J&J’s COVID-19 shot works

The Johnson & Johnson vaccine uses a different method to prime the body to fight off COVID-19 : a viral vector called Ad26. Viral vectors are common viruses that have been genetically altered so that they do not cause illness but can still cause the immune system to build up its defenses. The Pfizer-BioNTech and Moderna vaccines use messenger RNA to do that.

How well it works

The Johnson & Johnson vaccine is rated as highly effective at preventing serious illness and death, as the Pfizer-BioNTech and Moderna vaccines are. It is also very effective at preventing milder illness, though a bit less so than those two. It appears to do well against the highly contagious B.1.351 variant, first identified in South Africa, that has given at least one other vaccine candidate trouble.

Storage and handling

The Johnson & Johnson vaccine does not have to be stored at extremely low temperatures like the Pfizer-BioNTech vaccine. It can safely be kept in an ordinary refrigerator for three months, much longer than the Moderna vaccine, which spoils after a month if not kept frozen.

Side effects

The Johnson & Johnson vaccine appears to be less prone than the Pfizer-BioNTech and Moderna vaccines to trigger the kinds of side effects that require monitoring after the injection, which may make it more suitable for use at drive-thru vaccination sites. There have been reports that side effects tend to be felt more strongly after second doses, which the Johnson & Johnson vaccine does not require.

Patrick J. Lyons. c.2021 The New York Times Company