I recently saw a cartoon of a man wearing a mask running into a bank in 2019 with the staff shouting ‘call security’. The same picture in 2020 shows a man running into a bank without a mask and the staff shouting ‘call security’.

After tomorrow (July 24), we must all wear masks in shops or any public area where we cannot keep two metres apart; so why the change of policy?

The Conservative MP Sir Desmond Swayne claimed this is ‘a monstrous imposition’ but a YouGov poll found that the ruling has widespread public support.

There are very few trials on the importance of wearing masks but sometimes we cannot rely on a trial. We have to use other evidence. There has never been a controlled trial proving the safety of parachutes with one group jumping from a plane with a parachute and a control group without.

With a new drug we give one group of patients the drug and another group an alternative and measure the results. Neither group knows whether they are taking the active drug or not.

But how can we do this for masks? We cannot give one group of shoppers masks and another group no mask and not tell them. It would also be unethical to tell people to go into an epidemic without a mask to see what happens.

Countries where most people wear masks, particularly in the Far East, have done well. Deaths from coronavirus in 198 counties where it was Government policy or the cultural norm to wear face masks had lower death rates.

But was this due to the masks or are the people who wear masks more likely to be careful in other ways?

Research has shown that hundreds of potentially infected droplets are released during normal speech. Many of these are blocked by a simple cloth covering the mouth and nose. Professional clinical face masks need to be kept for those on the front line.

But is there any evidence in the real world? When masks were made compulsory in 15 states in the US District of Columbia the daily growth rate of Covid-19 slowed within three weeks.

In Germany, different states introduced masks at different times. In Jena on April 6 facemasks were made compulsory in shops; and new cases plummeted. On April 20, Saxony followed with the same results. All over Germany whenever masks were made compulsory cases fell.

There is also anecdotal evidence.

In one case a man tested positive for coronavirus after a flight from China to Toronto. He wore a mask for the whole flight. None of the 25 people closest to him on the flight tested positive.

Two hairdressers in Missouri were found to be positive after treating 140 clients. Everyone had worn a mask and all the clients tested negative.

There is less evidence that masks protect the wearer from catching the disease but even here evidence is building up.

This virus has not gone away and could become a major problem again this winter.

We know that people can have the virus with no symptoms. They may unwittingly spread it. We cannot look at a crowd and say who has the virus and who is clear. Masks will reduce the risk of spread.

Hand washing and social distancing are still important. A mask does not mean we can ignore all the other advice.

If you are foolish enough not to wear a seat belt in a car or crash helmet on a motor bike you put yourself a risk. If you fail to wear a mask you are putting other people at risk.

There is another advantage. The Father’s Day card from one of our sons said ‘Dad, I bought you a gag, face mask for protection from your jokes’.