Study shows one vaccine dose cuts Covid transmission by half

Researchers have some good news even as cases of Covid-19 continue to surge globally.

They have found that one dose of Covid-19 vaccines can cut coronavirus transmission by up to half.

A study done by Public Health England shows people who have been vaccinated with either AstraZeneca’s ChAdOx1 nCoV-19 or Pfizer’s BNT162b2 are between 38 per cent and 49 per cent less likely to pass the virus on to the people they stay with at the household level, as compared to those who are unvaccinated.

The researchers evaluated 552,984 households of two to 10 people where at least one person had been vaccinated prior to a positive test.

They compared the likelihood of SARS-CoV-2 transmission for vaccinated individuals with unvaccinated ones.

In the study published in the preprint server, they found that protection against Covid-19 started to be seen from around 14 days after the vaccination, regardless of age.

The people under observation had been vaccinated 21 days or more prior to testing positive for Covid-19 and the results showed “the likelihood of household transmission was 40-50 per cent lower for households in which the index cases were vaccinated 21 days or more prior to testing positive, as compared to no vaccination, for both ChAdOx1 nCoV-19 and BNT162b2 vaccine.”

According to the researchers, the effect on onward transmission at the household level was predominantly obvious for index cases aged less than 70 years. They observed that the trend was the same for secondary infections in household contacts.

The results on timing of vaccination show reduction in transmission was detected at 14 days after vaccination, “which is consistent with the timing of effective protection from infection for the vaccinated individual.”

"Terrific news"

The investigators used three different analytical approaches to compare index cases vaccinated 21-35 days before a positive test with index cases vaccinated one to 10 days before a positive test.

All the approaches showed reductions in transmission for vaccination 21 to 35 days before a positive test with both the AstraZeneca and Pfizer vaccines.

“These results lend confidence to the overall conclusion that vaccination reduces transmission, as the comparison is restricted to index cases that have received a particular vaccine, varying only in the timing of vaccination before testing positive,” the researchers said.

Speaking about the study, United Kingdom’s Health and Social Care Secretary Matt Hancock said, “This is terrific news, we already know vaccines save lives and this study is the most comprehensive real-world data showing they also cut transmission of this deadly virus.”

Covid-19 cases have fallen significantly in the UK. This has been attributed to the vaccination exercise, with the research showing that besides protection, there was a reduced risk of a vaccinated person developing symptomatic infection at around 60 to 65 per cent in four weeks after receiving one dose of either vaccine.

In Kenya, 865,897 people had been vaccinated with the first dose of AstraZeneca vaccine as of Friday, of which 502,893 were aged 58 years and above. According to the Ministry of Health, 156,344 health workers, 134,083 teachers and 72,581 security officers had been vaccinated.

The Ministry of Health has already distributed a total of 1,099,000 doses of the 1.12 million doses of the AstraZeneca vaccine received into the country in March.

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