Pfizer-BioNTech is dominant because it completed its clinical trials fastest, smoothed out manufacturing problems, and expanded fastest from adults to children and from emergency authorization to full approval. These are all lessons from commercial drug launches.

J&J, by contrast, had disadvantages from the start. It launched its study later, and it may have chosen a one-dose approach at first because doses of its adenovirus-based vaccine couldn’t be given too soon after one another. (Pfizer’s vaccines are given three weeks apart; for its two-dose study, J&J chose a two-month interval.) Stoffels also thought that in a pandemic, a one-dose shot with lower efficacy would be preferred to a two-dose one. But in the end, people preferred the vaccine with the better efficacy.

This situation is not ideal, because mRNA vaccines by themselves might not be the best solution for fighting Covid over the long haul. Covid-19 isn’t going away, and it might not be wise to rely so heavily for the world to rely on the first vaccine technology that worked. It might be that, as J&J seems to believe, its vaccine or AstraZeneca’s might give broader protection against still unseen variants. Protein-based technologies like Novavax’s might have fewer side effects if people do need annual boosters. Nasally delivered vaccines might provide different types of protection.