One Dose of COVID-19 Vaccines Halves Onward Transmission, Study

A woman receiving CCP virus vaccination on Jan. 23, 2021. (PA)

One dose of either Pfizer/BioNTech or AstraZeneca vaccines can reduce the risk of CCP virus transmission by up to a half, a new study has found.

A preprint of the study (pdf) by Public Health England (PHE) said that those who received one dose of a vaccine at least 21 days before being tested positive have a 40–50 percent lower chance of transmitting it within the same household than those who were unvaccinated.

The study also said the data indicated that the reduction in transmission can be detected at 14 days after vaccination.

PHE analysed data from 365,447 households with one person that has tested positive for COVID-19, the disease caused by the CCP (Chinese Communist Party) virus, between Jan. 4 and Feb. 28 (index case).

All index cases were aged 16 and over, and most of them are under 60 years old. Among them, most unvaccinated index cases were under 40—the age group that’s low on the government’s priority list.

Order of priority list for COVID-19 vaccination roll-out in England. (PA Graphics)

The study observed the data from these households for 14 days after the index case was tested, and compared the chance of a second person being tested positive (secondary case) within the households.

Vaccinated household contacts are excluded from this study, meaning all secondary cases were unvaccinated.

Households have been consistently identified as being at high risk for secondary infection. PHE said the result of the study could also have implications for transmissibility in other settings.

“This protection is on top of the reduced risk of a vaccinated person developing symptomatic infection in the first place, which is around 60 to 65 [percent]—4 weeks after one dose of either vaccine.” the government said in a statement.

Health Secretary Matt Hancock said the news is “terrific.”

“I urge everybody to get their vaccines as soon as they are eligible and make sure you get your second dose for the strongest possible protection,” Hancock said.

Dr. Mary Ramsay, head of immunisation at PHE, also encouraged people to take their vaccines as soon as they are offered.

“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,” Ramsay said while asking people to remain cautious.

“While these findings are very encouraging, even if you have been vaccinated, it is really important that you continue to act like you have the virus, practise good hand hygiene and follow social distancing guidance,” she said.

Retired communicable disease control consultant Dr. Peter English said that the numbers analysed in the study are “substantial” and the conclusions are “robust.”

English said the study may have underestimated the effectiveness of the vaccines in preventing transmission, “as their definition of secondary cases is likely to have included some co-cases (people who acquired their infection from the same source as the ‘index case,’ rather than from the index case).”