When will we need booster shots? And which ones?

Possibly. Scientists have already found that vaccines using different technologies can vary in their effectiveness. The strongest vaccines include Moderna and Pfizer-BioNTech, both of which are based on RNA molecules. Vaccines relying on inactivated viruses, such as those made by Sinopharm in China and Bharat Biotech in India, have proved somewhat less effective. It’s not entirely clear why that’s the case, said Scott Hensley, an immunologist at the University of Pennsylvania. RNA vaccines are relatively new and so the immunity they provoke has not been thoroughly studied. In his own research on mice getting different types of flu vaccines — some made with RNA and others inactivated viruses — Dr. Hensley sees a similar difference. The level of antibodies produced by the two kinds of vaccines are “outrageously different,” he said. It’s possible that the protection from the less effective Covid-19 vaccines will fade more quickly. Sinopharm’s vaccine may already be showing some signs of this decline. Clinical trials indicate that it has an efficacy of 78 percent. But the United Arab Emirates and Bahrain are already offering boosters to people who received the Sinopharm vaccine to bolster their waning immunity.