The coronavirus crisis led to a huge drop in the numbers of people going to accident and emergency units in England last month compared with March last year, official figures show.
At the same time, though, calls to NHS 111 rose to nearly three million, twice as many as in the same month in 2019.
Minsters say people must continue to stay at home to stop the virus.
But the Health Foundation said they should still feel able to seek treatment for serious conditions.
“Further work is needed to understand who is not coming to A&E and whether unmet needs are being stored up for the future,” said Tim Gardner, senior policy fellow at the think tank.
There is also evidence the number of people seen in hospital in England