A new study from the University of Oxford supports this: people with large amounts of the virus in the body have not transmitted it like the unvaccinated. Measuring viral load overestimates how infectious fully vaccinated people are.

One of the biggest concerns about living with the Delta variant (now prevalent) is that the vaccinated if infected, have the same ability to transmit the virus as the unvaccinated.

Even with the same viral load, it is less infectious.

It is a hypothesis that had been made by measuring the viral loads of individuals infected with Delta, which had been found to be very high. This dynamic has often been used as an argument for no vax in debates, but had been disproved over time by case numbers. The relative scientific studies, however, were still few.

Now a pre-print of an important study on this subject has come out from the University of Oxford. The research, dated Sept. 29 and not yet reviewed, says vaccinated people are less infectious even with Delta and even if the CT cycle count (which measures viral load) is the same.

In addition, it shows that children appear to be less infectious and less susceptible, even with Delta, just as they did with the other variants.

The data

The data was derived from a retrospective observational cohort study of SARS-CoV-2 contacts infected by the ‘index cases’ (the first infected of an outbreak) using swab data from England between January 2021 and August 2021.

The “second infections” occurred mainly by contact within families (70%), but also after home visits (10%), events or activities (10%) and at work or school (10%). 

The results showed that two doses of Pfizer or AstraZeneca in Alpha variant index cases reduced PCR positivity in contacts. Even with the Delta variant, the vaccines attenuated the associated infections: two doses of Pfizer reduced transmission more than with AstraZeneca. 

Protection from infection has decreased over time after the second dose, with Delta returning to levels similar to the unvaccinated within 12 weeks with the AstraZeneca vaccine and for Pfizer, there was a substantial decrease in protection after that time, but not so drastic.

Viral Load Does Not Measure Infectivity

Delta variant infections had higher viral loads than Alpha in both symptomatic and asymptomatic infections, regardless of vaccination status. The viral load of vaccinated and unvaccinated was similar. This did not lead to the same likelihood of infecting: most of the protective effect of the vaccines remained, that is, the study showed that other factors are important in reducing vaccine-associated transmission.

For example, the authors of the article write, “it is possible that vaccination facilitates a more rapid elimination of viable infectious virions, leaving behind ineffective and damaged virions that still contain RNA detectable by PCR (molecular swab”).

Measuring viral load, therefore, overestimates how infectious vaccinated people are. 

This suggests more than anything else greater infectivity of the virus itself, not only as a factor in the dynamics of the viral load: vaccinated people are not as contagious as unvaccinated people infected with Delta, even with the same viral load.

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