Thread by @sailorrooscout

A 50,000+ study out of the University of Oxford shows while “breakthrough” infections with Delta have similar viral loads (Ct) to unvaccinated individuals, vaccination DOES REDUCE transmission of Delta and vaccinated individuals are LESS LIKELY to spread the Coronavirus. 🧵

Pre-Delta, vaccination reduced transmission of SARS-CoV-2 from individuals infected despite vaccination, potentially via reducing viral loads. While vaccination still lowers the risk of infection, similar viral loads in vaccinated and unvaccinated individuals infected with Delta

question how much vaccination prevents onward transmission. Researchers here performed a retrospective observational cohort study of contacts of SARS-CoV-2-infected index cases using contact testing data from England. They used multivariable logistic regression to investigate the

impact of index case and contact vaccination on transmission, and how this varied with Alpha (B.1.1.7) and Delta (B.1.617.2) variants (classified using S-gene detection/calendar trends) and time since second vaccination. So what did they find? A couple of really important things.

51,798/139,164 contacts tested were PCR-positive. Two doses of Pfizer’s or AstraZeneca’s vaccines in Alpha variant index cases independently reduced PCR-positivity in contacts. The Delta variant attenuated vaccine-associated reductions in transmission: two doses of Pfizer reduced

Delta transmission slightly more than AstraZeneca’s. Variation in viral load (Ct values) explained ONLY a modest proportion of vaccine-associated transmission reductions. THIS is important. Why? BECAUSE CT VALUES ARE NOT A MEASURE OF INFECTIOUSNESS.

The authors state: “Factors OTHER than PCR-measured viral load are important in vaccine-associated transmission reductions.'' Let’s continue. Transmission reductions did slightly decline over time HOWEVER the effects on reducing susceptibility to infection appeared stronger and

remained well preserved, despite some waning. WITH THIS SAID, there was an age dependent effect. Researchers state some of the observed decline may be attributable to the fact that those clinically vulnerable with weaker immune systems were vaccinated longer ago, which is why

boosters are beneficial for those who are most high-risk as they will help control transmission AND help prevent infections. Researchers also found there were much lower rates of secondary infection from child index cases AND lower secondary infection in child contacts.

So what does this tell us? Firstly, this is ONLY pertaining to breakthrough infections in vaccinated individuals. Why is this important? Because this only applies to WHEN and IF they happen! Preventing infection IS preventing transmission as well. The vaccines do a very good job

of preventing you from facing symptomatic infection in the first place! Listen to me clearly here so that you can pass it on as well: vaccinated individuals are not more likely to transmit to others. We know they can transmit however, they are indeed LESS likely to get infected

in the first place and therefore also LESS likely to transmit to others. This study shows us that while viral load may be the same, vaccinated individuals who DO get infected are LESS likely to be infectious than unvaccinated individuals with a naive infection.

Their conclusion? Vaccination reduces transmission of Delta, but by less than the Alpha variant. Vaccination also reduces virus shedding time, prevents symptomatic infection, & the presence of an immune response elicited by the vaccines will ultimately suppress the viable virus.

You can find this study here: medrxiv.org/content/10.110… I also HIGHLY recommend this fantastic article: No, Vaccinated People Are Not ‘Just as Likely’ to Spread the Coronavirus as Unvaccinated People: theatlantic.com/ideas/archive/… as well as my previous threads of studies on this topic.