What we know about Covid transmission

Just one dose of the Pfizer-BioNTech or Oxford-AstraZeneca vaccines can cut transmission of Covid-19 by up to nearly 50%, a new study has found.

The Public Health England research found that people who had one dose of either of the jabs but became infected with the coronavirus at least three weeks later were between 38% and 49% less likely to spread it to other members of their households, compared with unvaccinated people.

Welcoming the “terrific news”, Health Secretary Matt Hancock said that the data “further reinforces that vaccines are the best way out of this pandemic”.

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How is Covid-19 transmitted?

Guidance from the World Health Organization (WHO) says the coronavirus can spread “from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe heavily”. These liquid particles are known as larger “respiratory droplets” or smaller “aerosols”.

If these get into another person’s mouth, nose or eyes, they can catch the virus. The WHO guidance, last updated in October, says the risk is greatest when people are in close contact, particularly when spending long periods of time together in “indoor, crowded and inadequately ventilated spaces”.

Is it transmitted on surfaces?

The WHO says the virus can also spread via contaminated surfaces such as tables, doorknobs and handrails.

However, Nature magazine says that after a year of gathering data, researchers believe it is “people, not surfaces, that should be the main cause for concern”.

“Surface transmission, although possible, is not thought to be a significant risk,” the magazine concludes.

How do vaccines help?

Experts had been awaiting data on whether vaccinated individuals could unknowingly pass the virus on to others despite having no symptoms themselves.

Clinical trials for vaccines such as the Pfizer-BioNTech jab did not look at their ability to block transmission, because “with thousands of people being hospitalised and dying every day, the first priority was to measure whether a vaccine prevented severe disease and death”, National Geographic explains.

But a consensus is beginning to emerge that the vaccines are having an impact on infection rates too.

The latest PHE study, which is yet to be peer-reviewed, looked at data on 57,000 people living in a total of 24,000 households who were in contact with a vaccinated person, compared with nearly one million contacts of unvaccinated people.

“Protection was seen from around 14 days after vaccination, with similar levels of protection regardless of age of cases or contacts,” said PHE in a statement.

Dr Peter English, a retired consultant in communicable disease control, said: “The evidence was already mounting that vaccination will prevent people from becoming infected, and if they aren’t infected, they can’t transmit the infection.”