Concerns about blood clots with Johnson & Johnson underscore just how lucky Americans are to have the Pfizer and Moderna shots.
A year ago, when the United States decided to go big on vaccines, it bet on nearly every horse, investing in a spectrum of technologies. The safest bets, in a way, repurposed the technology behind existing vaccines, such as protein-based ones for tetanus or hepatitis B. The medium bets were on vaccines made by Johnson & Johnson and AstraZeneca, which use adenovirus vectors, a technology that had been tested before but not deployed on a large scale. The long shots were based on the use of mRNA, the newest and most unproven technology.
The protein-based vaccines have moved too slowly to matter so far. J&J’s and AstraZeneca’s vaccines are effective at preventing COVID-19—but a small number of recipients have developed a rare type of blood clot that appears to be linked to the adenovirus technology and may ultimately limit those shots’ use. Meanwhile, with more than 180 million doses administered in the U.S, the mRNA vaccines have proved astonishingly effective and extremely safe. The unusual blood clots have not appeared with Pfizer’s or Moderna’s mRNA technology. A year later, the risky bet definitely looks like a good one.