| The number of vaccines against severe Covid-19 is poised to grow, as Johnson & Johnson and Novavax both announced positive results last week. Their looming authorization also raises the possibility of choice: Which vaccine do you want? The answer, and the reasons behind it, might not be what you expect. Neither of the new approaches appears as effective as the first shots that use a new technology called messenger RNA. Those—one from Pfizer and its partner BioNTech, the other from Moderna—prevented about 95% of symptomatic cases. The Novavax shot prevented 89%, and J&J's shot was 66% effective. All four, plus vaccines that aren't available in the U.S. from AstraZeneca, and those out of China and Russia, surpassed the 50% effectiveness level set by U.S. regulators for authorization. So should you hold out for the vaccine with the best numbers? Not necessarily. These aren't therapies that you take to treat a disease. No one would take an antibiotic that's 66% effective if one that's 95% effective is available. But people getting immunized aren't sick. Hopefully, they will never need that shot to do anything at all.  Doses of Pfizer-BioNTech Covid-19 vaccine being prepared for injection last month. Photographer: KENZO TRIBOUILLARD/AFP Vaccines should be considered more of an insurance policy, one that you don't ever want to use. Like car insurance, you want everyone to have it. The highest-risk people should get the most protection, since they are most likely to need it. That is key in a pandemic of respiratory disease, when a virus moves quickly from one person to the next, constantly mutating in ways that may maximize its efficiency in infecting and spreading. Vaccines put up hurdles, of various heights, by making it harder for the virus to take hold in a potential new victim. If the vaccines can block enough of the spread, on a broad scale, it will be harder for the virus to compete and eventually it will be out of the race. With less virus circulating, there will be less risk of exposure for everyone, whether they got the most powerful vaccines, less powerful ones or none at all. The good news with both the J&J and Novavax shots is their ease of distribution; they don't need to be in a deep-freeze during transportation. The J&J immunization works with just a single shot. Both companies have pledged to produce millions of doses for the world in the coming months. So which shot should you get? The first one offered. Don't cut in line. But may you get it soon, and need it never.—Michelle Fay Cortez | 
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