The Trump administration has delivered its latest blow to vaccines, canceling a nearly $600-million contract to the drugmaker Moderna that was intended to develop a shot for humans against bird flu.

The cancelation came as Moderna announced positive interim results from an early-stage trial of the vaccine that targeted H5 bird flu virus, tested in 300 healthy adults.

Donald Trump once again got testy with a reporter over a question, this time over a phrase being used on Wall Street over the president’s tendency to not follow through on tariff threats: the TACO tra...