Study: Vaccinated People Less Contagious Despite Similar Ct Count As Unvaccinated

Two doses of the Pfizer and AstraZeneca Covid-19 vaccines can reduce transmission of the Delta variant by 65% and 36% respectively, a new UK government-funded study shows.

KUALA LUMPUR, Oct 1 — A new UK government-funded study is proving that Covid-19 vaccination is effective at reducing Delta transmission, even if vaccinated people have similar viral loads as the unvaccinated. 

A preprint of the study posted on medical news site medRxiv recently showed that two doses of the Pfizer-BioNTech and AstraZeneca-Oxford coronavirus vaccines reduced transmission of the Delta variant by 65 per cent and 36 per cent, respectively.

Using a data sample of 139,164 Covid-19 contacts in the UK, the study found that vaccines were effective at reducing transmission of the Delta variant, despite vaccinated people showing similar cycle threshold (Ct) values as unvaccinated individuals. Ct values may indicate the amount of virus carried by a person.

The effectiveness of the vaccines in reducing Delta transmission, however, is slightly less compared to the Alpha variant where individuals fully vaccinated with the Pfizer and AstraZeneca vaccines are 82 per cent and 63 per cent less likely to transmit the virus.

The study also found that transmission reductions wane over time, with the odds of contracting the virus rising by 1.2-fold for the Pfizer vaccine or 1.13-fold for the AstraZeneca vaccine every two weeks, 14 days after the second dose.

Multiple other factors were also associated with contacts testing positive, including contact event type and index case age, researchers noted.

The highest rates of PCR-positivity after household contact with index cases were aged 30 years and above and the lowest rates following contact with index cases were below 20 years.

Contacts in their 30s, 40s and 70s had the highest rates of positive tests after household contact, while contacts in their 20s had the highest rates after contact events outside their own homes. This suggests that children are less infectious and less susceptible to the virus.

Researchers concluded by suggesting that booster vaccinations may help control transmission together with preventing infections.

The UK study points to growing evidence that proves that vaccines substantially reduce the risk of passing on the virus.

Two studies from Israel posted as preprints in July found that two doses of Pfizer are 81 per cent effective at preventing Covid-19 infections. Vaccinated people who do get infected are 78 per cent less likely to spread the virus to household members than unvaccinated individuals.

In July, internal documents by the United States’ Centers for Disease Control and Prevention (CDC) sighted by The Washington Post suggested vaccinated people infected with the Delta variant of Covid-19 may transmit the virus as much as unvaccinated individuals.

The CDC’s internal document noted that vaccines prevent more than 90 per cent of severe disease, but may be less effective at preventing infection or transmission, thus more breakthrough infections and higher community spread may occur despite vaccination.

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