The British government on Friday shut down or scaled back a number of its COVID-19 surveillance programs, curtailing the collection of data that the United States and many other countries had come to rely on to understand the threat posed by emerging variants and the effectiveness of vaccines. Denmark, too, renowned for insights from its comprehensive tests, has drastically cut back on its virus tracking efforts in recent months.

As more countries loosen their policies toward living with COVID-19 rather than snuffing it out, health experts worry that monitoring systems will become weaker, making it more difficult to predict new surges and to make sense of emerging variants.

"Things are going to get harder now,” said Samuel Scarpino, a managing director at the Rockefeller Foundation’s Pandemic Prevention Institute. "And right as things get hard, we’re dialing back the data systems.”