As scientists try to track the spread of a new, more infectious coronavirus variant around the world — finding more cases in the United States and elsewhere this week — they are also keeping an eye on a different mutation with potentially greater implications for how well Covid-19 vaccines work. The mutation, identified in a variant first seen in South Africa and separately seen in another variant in Brazil, changes a part of the virus that your immune system’s antibodies get trained to recognize after you’ve been infected or vaccinated.