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Vaccination associated with a smaller reduction in transmission of the Delta variant than Alpha form

Vaccination against COVID-19 still enables a higher level of transmission of the Delta variant compared with the Alpha variant

Vaccination has been found to produce a smaller reduction in the level of transmission for the Delta compared with the Alpha COVID-19 variant according to a study by researchers from Oxford University, UK.

The use of vaccines against COVID-19 has been shown to reduce symptomatic infection and even onward transmission of the virus among household contacts. In addition, other work has found that this reduced risk of onward transmission, is primarily due to the presence of a lower viral load among vaccinated individuals. Nevertheless, other evidence has revealed a similar viral load among those who are vaccinated but infected with the Delta variant.

For the present study, the team from Oxford turned their attention to national contact testing data in England and looked for adults (> 18 yeas of age) with both symptomatic and asymptomatic infections. Their analysis included vaccination with either BNT162b2 or ChAdOx1 to investigate differences in transmission from index patients infected with either variant. The team developed regression models to determine any associations between onward transmission and the vaccination status of the index patient.


Among a total of 146,243 tested contacts from 108,498 index patients, with a median age of 34 years (51% female), 37% had a positive PCR test.

Using regression modelling, among index patients doubly vaccinated with BNT162b2 and who became infected, there was a significantly reduced risk of onward transmission of the Alpha variant (adjusted rate ratio, aRR = 0.32, 95% CI 0.21 – 0.48) compared unvaccinated individuals. Similarly, those with two vaccination doses of ChAdOx1, also had a reduced risk of onward transmission (aRR = 0.48, 95% CI 0.30 – 0.78) compared to the unvaccinated.

In contrast, while the extent of onward transmission of the Delta variant was also reduced compared to Alpha by both vaccines, it was greater among those doubly vaccinated with BNT162b2 (aRR = 0.50, 95% CI 0.39 – 0.65) compared to ChAdOx1 (aRR = 0.76, 95% 0.70 – 0.82). In other words, index patients vaccinated with BNT162b2 were less likely to have contacts with a positive PCR test for Delta compared to those given the ChAdOx1 vaccine.

The Delta variant was also associated with more onward transmission from both symptomatic index patients (aRR = 1.24, 95% CI 1.12 – 1.38) and from asymptomatic individuals (aRR = 1.40, 95% CI 1.22 – 1.59) and this was independent of both index and contact vaccination status.

Interestingly, the risk of infection with the Alpha variant among fully vaccinated contacts, was much lower among those given BNT162b2 (aRR = 0.15, 95% CI 0.11 – 021) compared to those fully vaccinated with ChAdOx1 (aRR = 0.40, 95% CI 0.27 – 0.59) and the magnitude of these reductions were similar for infections with the Delta variant.

Both symptomatic and asymptomatic index patients infected with the Delta variant had lower Ct values (i.e., higher viral loads) compared to those infected with the Alpha variant. When including Ct values in their regression models, the authors reported that lower Ct values were independently associated with increased transmission of either variant.

The authors concluded that vaccination was associated with a smaller reduction in transmission of the Delta compared to the Alpha variant.


Eyre DW wt al. Effect of Covid-19 Vaccination on Transmission of Alpha and Delta Variants. N Engl J Med 2022

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