Medical Testing

The most widely used coronavirus vaccines are designed as two-shot inoculations, and nearly everyone worldwide who has had both doses has received the same vaccine both times.

But that is changing, as more countries are allowing — and even, in some cases, encouraging — mix-and-match inoculation, with people receiving a first shot of one vaccine, and then a second shot of a different one. On Tuesday, Germany’s government revealed that Chancellor Angela Merkel had received two different shots, adding to the growing interest in the practice.