Even 1 dose of COVID-19 vaccine could cut transmission almost in half

A single dose of COVID-19 vaccine using drugs recommended for two doses for maximum immunity can cut household transmission of coronavirus by up to half, a new study has suggested. The research, carried out by the UK's Public Health England (PHE), looked at how likely it was that COVID-19 could be passed between people living in the same home after even partial vaccination had been given.

PHE looked at two COVID-19 vaccines, Pfizer BioNTech's and AstraZeneca's. Of the two, only the former is currently in use in the US, though both drugs are being distributed in the UK and elsewhere. Each recommends two doses, given roughly a month apart, in order for individuals to see maximum immunity from severe cases of coronavirus that would otherwise likely result in hospitalization or even death.

What the vaccines don't do, however, is prevent people from catching COVID-19 at all. Instead, they're intended to reduce the severity of those infections, so that they don't become life-threatening. Some people who have caught COVID-19 after being vaccinated have reported it feels more akin to a heavy cold or flu, though experiences vary.

A lingering question, meanwhile, has been around the impact of vaccinations on COVID-19 transmission. With the immunization process still underway, large swathes of the population still remain unvaccinated; it'd been unclear just how likely vaccinated people are to pass coronavirus to that unvaccinated group. Exacerbating the confusion is that asymptomatic COVID-19 – where no symptoms are shown, but the individual can still be infectious nonetheless – seems to be more likely than symptomatic cases among the vaccinated group.

"This new research shows that those who do become infected 3 weeks after receiving one dose of the Pfizer-BioNTech or AstraZeneca vaccine were between 38% and 49% less likely to pass the virus on to their household contacts than those who were unvaccinated," the PHE said today of its study. "Protection was seen from around 14 days after vaccination, with similar levels of protection regardless of age of cases or contacts."

Meanwhile, the organization says, vaccinated people are at reduced risk of developing a symptomatic COVID-19 infection to begin with. That's said to be around 60-65 percent from four weeks after the first dose of either of the two drugs.

"Households are high-risk settings for transmission and provide early evidence on the impact of vaccines in preventing onward transmission," the PHE added. "Similar results could be expected in other settings with similar transmission risks, such as shared accommodations and prisons."

Public Health England looked at over 57,000 contacts from 24,000 households, in which there was a lab-confirmed case that had received a vaccination. That's compared with nearly 1 million contacts of unvaccinated cases.

"Not only do vaccines reduce the severity of illness and prevent hundreds of deaths every day, we now see they also have an additional impact on reducing the chance of passing COVID-19 on to others," Dr Mary Ramsay, Head of Immunization at PHE, said of the findings. "I encourage anyone who is offered a vaccine to take it as soon as possible."

The US CDC announced changes to its guidelines for fully vaccinated people – that is, people who have waited two weeks after the final dose of their vaccine, whether that be a single-dose or two-dose drug – and unvaccinated people when outdoors. It's now considered safe for fully vaccinated and unvaccinated people to do outdoor activities like walking and biking without wearing masks, the CDC suggested, though indoor events can still be a significant risk.