Vaccines reduce risk of Delta infection in the home by about a third

Vaccination only reduces the risk of infection with the Delta variant of coronavirus by about a third, according to a detailed study of transmission within households.

The research showed that the chance of contracting Delta, which has established itself as the dominant strain of the virus over the past six months, from someone at home was 25 per cent for those double-jabbed, compared with 38 per cent if you were unvaccinated. Even this limited protection begins to wane three months after vaccination, the study led by Imperial College London found.

The scientists found that the risk of onward transmission was little affected by vaccination status of the infected person with peak levels of virus similar in people who had and had not been jabbed.

“Vaccines are critical to controlling the pandemic, as we know they are very effective at preventing serious illness and death from Covid-19,” said Ajit Lalvani, professor of infectious diseases at Imperial. “However, our findings show that vaccination alone is not enough to prevent people from being infected with the Delta variant and spreading it in household settings.”

The study, funded by the National Institute for Health Research and published in The Lancet Infectious Diseases, adds details about transmission within households to the evidence emerging from large-scale epidemiological data about the protection provided by vaccines against infection. Most research has so far focused on the impact on hospitalisation and death, with studies finding the main vaccines developed by western scientists to be more than 90 per cent effective.

Other research looking at the impact of vaccines on infection has suggested greater effectiveness in preventing the spread of the Delta variant than the Imperial study. But the Imperial researchers said these may underestimate the real transmission rates by testing at a single point and relying more on the reporting of symptoms.

The researchers enrolled 231 household contacts of 162 people who had tested positive for the Delta variant but had mild or no Covid symptoms. They were then tested daily for at least two weeks, starting within five days of exposure, to determine levels of virus in their nose and throat.

The study found vaccination status made little difference to the way viral load rose to a peak a few days after exposure when people are most infectious. But it did conclude that the immune systems of those who were double-jabbed cleared the infection more quickly than in unvaccinated people, leading to a faster decline from the peak in the amount of virus in the upper respiratory tract and greatly reducing the risk of severe illness.

The study, however, also found that the protection against infection provided by vaccination started to decrease soon after the second jab had taken effect.

But Lalvani said this did not necessarily mean that the UK should cut the recommended six-month interval between the second dose and a third booster jab. “Although we are seeing evidence of waning immunity after three months, it is important to focus first on six months and make sure that is happening.”

The analysis did not distinguish between different vaccines. Nor was there enough data to compare the effect of double-jabbing on earlier variants such as Alpha.

Rowland Kao, professor of epidemiology at Edinburgh university, who was not involved in the study, said it highlighted “the roles that households play in spreading Covid — long-term close contact indoors being perhaps the most important risk factor for transmission”.

Lalvani said the key lesson was that “unvaccinated people cannot rely on the immunity of the vaccinated population for protection”.