jab thread

Scientific teams around the world have developed successful Covid-19 vaccines in incredibly short order. In a feat that even a couple of years ago would have seemed completely out of reach, vaccines to protect against the new disease were being used before the first anniversary of the disclosure that a new threat existed. This is truly extraordinary.

In the United States, three vaccines are available. Two — those from Moderna and Johnson & Johnson — have emergency use authorizations, which are green lights from the Food and Drug Administration to be put into use, even though they have not yet been fully licensed. In August 2021, the FDA gave full approval to the vaccine developed by the partnership of Pfizer and German manufacturer BioNTech for people 16 and older.

The Pfizer shot first received emergency authorization in mid-December, followed closely by the one developed by Moderna with assistance from the National Institute of Allergy and Infectious Diseases.

In late February came a third. Janssen Pharmaceuticals, the vaccine division of Johnson & Johnson, secured an emergency use authorization for its one-dose vaccine. The vaccine was seen as a potential game-changer. It doesn’t require the fussy cold-chain needed to keep the Pfizer and Moderna vaccines from spoiling. It is cheaper, both on a per-dose basis and because only one dose is needed. Plus it was thought a one-and-done option might entice some people who couldn’t easily get two doses to roll up their sleeves.

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But since the J&J vaccine’s arrival on the scene there have been a number of challenges. A production snafu in the hands of a contract production company contaminated 15 million doses, which had to be destroyed. And in mid-April, the FDA and CDC recommended states pause use of the vaccine as they investigate whether the vaccine triggers a rare but serious side effect — the development of diffuse blood clots, even though the few individuals who developed the condition had low platelet levels.

What follows is a head-to-head comparison of the vaccines developed by Pfizer and BioNTech, by Moderna, and by J&J. This article will be updated as developments occur.

Vaccine types

The Pfizer and Moderna vaccines are made using messenger RNA, or mRNA, a technology that delivers a bit of genetic code to cells — in effect, a recipe to make the surface protein (known as spike) on the SARS-2 virus. The proteins made with the mRNA instructions activate the immune system, teaching it to see the spike protein as foreign and develop antibodies and other immunity weapons with which to fight it.

The J&J vaccine uses a different approach to instruct human cells to make the SARS-2 spike protein, which then triggers an immune response. It is what’s known as a viral vectored vaccine. A harmless adenovirus — from a large family of viruses, some of which cause common colds — has been engineered to carry the genetic code for the SARS-2 spike protein. Once the adenovirus enters cells, they use that code to make spike proteins. J&J employed this same approach to make an Ebola vaccine that has been authorized for use by the European Medicines Agency.

Related: The story of mRNA: How a once-dismissed idea became a leading technology in the Covid vaccine race

Target population

The Pfizer vaccine has been approved for use for people aged 16 and older, though its EUA also extends to people ages 12 to 15. Moderna’s has been cleared for use in people 18 and older, though the company is now testing its vaccine in 12- to 17-year-olds. J&J’s vaccine has been tested in people 18 and older, and that’s who it was authorized for. Until testing in children and younger teens is conducted, this vaccine won’t be available for use anyone under 18 years old either.

In August 2021, the FDA also authorized third doses of the mRNA vaccines for people who have certain immunocompromising conditions, amid growing evidence that they do not get adequate protection from the normal two-dose regimen of the Pfizer-BioNTech or Moderna vaccines.

Vaccine efficacy

The Pfizer and Moderna vaccines have shown astonishing — and essentially equivalent — degrees of efficacy, at least in the early stages after vaccination.

The Pfizer vaccine showed efficacy of 95% at preventing symptomatic Covid infection after two doses. The vaccine appeared to be more or less equally protective across age groups and racial and ethnic groups.