Bob Wachter Profile picture
Chair, UCSF Dept of Medicine. Career: What happens when a poli sci major becomes an academic physician. Author: "The Digital Doctor". Hubby/Dad/Golfer.

Jul 30, 2021, 13 tweets

The CDC document that was discussed in @washingtonpost today… is now on line:…
The WashPo shared it with me before publication for reactions, and I'm quoted. Some thoughts follow. (1/13)

My overall view: we knew much of what's in here, but there's some new information & analysis. In some cases it's brand new, in others it clarified something we knew before. In virtually all cases, the new stuff's a bit worse than I expected. Here are the key findings: (2/13)

1) Delta is much more infectious than the original: they estimate an Ro of 5-9, vs. the 2-3 for the original, which makes Delta "as transmissible as chicken pox." We've been estimating Ro of ~6 for Delta, or ~2x as infectious as original. It may be a bit worse than that.(3/13)

2) They believe the period of infectivity is longer (high levels of viral shedding for 18, rather than 13 days). This will raise questions about lengths of quarantine and isolation, as well as appropriate timing of testing.(4/13)

3) As we learned in the CDC announcement 2 days ago, they're finding equal viral loads in noses/mouths of people with breakthrough infections as in infected unvaxxed people. Likely means that breakthrough cases can spread the virus more readily than we previously thought.(5/13)

4) Vaccine efficacy: several estimates; they tend to show ~80% efficacy vs. Delta (vs. the prior 95%). Thankfully vax IS still ~95% effective vs. serious infections. That the efficacy of the vaccines is holding up for serious infections is the main piece of good news here.(6/13)

5) Lower efficacy in selected populations: significantly lower efficacy (~70%) in nursing home elderly (Fig L) and in immunocompromised (Fig R) patients. These are likely to be the first groups made eligible for boosters. Hopefully soon.(7/13)

6) Waning of immunity: no new data here (that I saw) supporting Pfizer findings that mRNA efficacy wanes over time. But final bullet says protection from prior infection wanes after 6 months – adding to risk to those whose immunity was from prior Covid. They still need vax.(8/13)

7) Seriousness of Delta infections. Early UK/India studies pointed to more serious infections, then consensus seems to be that they were not more serious. This document supports the "more serious" stance. That's concerning.(9/13)

8) Notwithstanding all the noise about breakthrough infections, the case for vaccination remains enormously strong: they show an 8-fold reduction in cases, and a 25-fold reduction in hospitalizations and deaths with vaccination.(10/13)

9) That said, as more people are vaccinated, there'll be more people in hospital and more dying w/ breakthru infections (even though vaccines work! – it's just math). Estimate here: 9% of hospitalized pts & 15% of deaths in vaxxed pts. Seems a little high, need to confirm.(11/13)

My overall take: As the document says, "the war has changed." Data argues that universal masking is critical to block spread of Delta – a more infectious, and possibly more serious virus – particularly if it's true that vaccinated folks can be part of the chain of spread.(12/13)

Document also makes clear that we need a new & far more aggressive vaccination strategy (w/ everyone masking indoors until we're there) if we're to get ourselves out of this new & unsettling stage. The foe has gotten better at its job, and so the war has indeed changed.(13/end)

Share this Scrolly Tale with your friends.

A Scrolly Tale is a new way to read Twitter threads with a more visually immersive experience.
Discover more beautiful Scrolly Tales like this.

Keep scrolling