A quarter of Mexico's population, or 31 million people, has been exposed to Covid-19, according to preliminary results of an official survey, with 70 per cent never showing symptoms.

“The figures show a great speed of spread,” Juan Rivera, general director of the National Institute of Public Health, told a news conference.

The survey, which used blood tests to detect antibodies, involved 9,400 households and was carried out between August and November, Riviera said.

Only 20pc had symptoms that pointed to a Covid-19 diagnosis while 10pc had only some symptoms, according to the findings of the survey.