2022-12-24 Bob Wachter covid practical actions

Bob Wachter (@Bob_Wachter): Covid (@UCSF) Chronicles, Day 1006 (!)
There’s an awful lot of Covid confusion & misinformation out there, particularly around these three topics: masks, vaccines, and home tests. Let’s look at them in today’s thread. (1/24)

To start, these statements are correct:
• Wearing the wrong mask, or wearing a mask incorrectly, doesn’t work
• Vaccination/boosters don’t work as well as they used to in preventing infection
• Home tests yield more false negative results than they used to.

But these statements are unambiguously wrong:
• Masks don’t work
• Vaccines/boosts don’t work
• Home tests don’t work

This is confusing enough that it’s an easy mark for those who want to spread misinformation. Here’s how I interpret the science to inform my decisions: (3/24)

First, masks: When I’m in a crowded indoor space, I wear a KN95. That’s also what I’ll wear in the hospital (unless a patient has Covid; then I’ll wear a fitted N95). A surgical mask if worn correctly works moderately well, but I wouldn’t trust it in a hi-risk environment. (4/24)

A cloth mask works less well, though better than nothing. https://www.cdc.gov/mmwr/volumes/71/wr/mm7106e1.htm Of course, if you wear a mask incorrectly it won’t work well. And, politics aside, mask mandates would help prevent Covid if folks wore the right masks, correctly. If not, they won’t work. (5/24)

Let’s turn to vaccines and boosting: The early evidence on vaccines was astounding – about 95% efficacy against all infections, and nearly 100% efficacy against severe infection (hospitalization and death). (6/24) https://www.nejm.org/doi/full/10.1056/nejmoa2034577

With time – mostly owing to the immune-evasive properties of successive variants – efficacy (in terms of protection against infection) has fallen. We now know that current boosters cut the risk of infection by only ~50%, and the effect only lasts for 2-3 months. That’s...(7/24)

... not nothing, particularly during times (ie now) of high prevalence. But more importantly, studies show that the fall bivalent booster reduces hospitalization risk by ~50% across all age groups https://www.cdc.gov/mmwr/volumes/71/wr/mm715152e1.htm , and by ~75% in folks >65... https://www.cdc.gov/mmwr/volumes/71/wr/mm715152e2.htm/ (8/24)

... and this effect lasts for >3 months. Moreover, we now know that vaccination lowers the risk of Long Covid – both by lowering the infection rate in those first few months, but also by lowering the rate of LC in those who get Covid. https://www.nytimes.com/article/long-covid-vaccines.html (9/24)

Given that the risk of the vaccine & booster is exceedingly small, my judgment is that the benefits far exceed the risks – certainly in older people & those with medical co-morbidities but also (given that the risk of Long Covid isn’t age-related) in younger folks as well.(10/24)

So, while it’s accurate to say that vaccination/boosters don’t work as well as they once did to prevent infection, saying “they don’t work” is wrong and – to the extent folks listen – dangerous. I received my bivalent boost in Sept, and my 30-ish year-old kids did as well.(11/24)

Last, home tests. In 20-21, home tests could be counted on to be positive at or near onset of symptoms. Once Omicron took over, it became clear that someone could test negative on Day 1 of symptoms yet still have Covid, and probably be infectious. (12/24) https://www.bmj.com/content/378/bmj-2022-071215

There’s no evidence that the performance of the home test has deteriorated with successive Omicron variants. But the situation is confusing, since studies that cite the “performance” of the home test compared with PCR invariably show high (30-70%) “false negative” rates. (13/24)

Why’s the PCR more sensitive than home antigen? Because the PCR picks up (& amplifies) a tiny amount of virus, & thus will be positive w/ levels of virus too low to infect others. In fact, we know that the average person w/ Covid will stay antigen (home test) positive… (14/24)

... for ~8 days, while PCR remains positive for about twice that long. So if you do a head-to-head comparison of the tests, the home test will appear to be 50% sensitive – seeming to “miss” about half the cases. But that’s mostly a feature, not a bug (no pun intended). (15/24)

Those extra 8-10 days of PCR positivity during the recovery phase aren’t particularly useful to know, since the amount of live virus is too low to infect anyone. (This is why we don’t use a negative PCR as the signal to end isolation; we use negative home antigen tests.) (16/24)

Rather, it’s at the onset of symptoms where home tests’ lower sensitivity might lead to trouble. At that stage, the PCR may be positive 1-2 days before the home test turns positive. And it’s possible that someone will be infectious during that brief window. (17/24)

Does this render the home test useless? Absolutely not. In fact, if I I wake up tomorrow with new symptoms consistent with Covid, I’ll do a home test. (By the way, the govt. has made more free ones available: https://www.covid.gov/tests)
What will I do with the results? (18/24)

• If +, I have Covid. I’ll isolate & start Paxlovid
• If -, I could still have Covid – I’ll mask up & plan to retest tomorrow
• If repeat test is -, I’ll assume I don’t have Covid (& probably test for flu)
• If repeat is +, I have Covid & will isolate & start Pax (19/24)

Since a single home test can be negative yet you still can be infectious, this also influences how you should interpret group testing before an event. In a region in which case rates are moderately high but not off the charts (such as in SF now, with 24 cases/100K/day),… (20/24)

... I’d be OK being maskless in a small group (eg, my 8-person poker crowd) if everybody tested negative on a rapid test, & we added ventilation. The testing doesn’t drop the risk to zero, but it lowers the risk by ~50-70% (compared with no testing), rendering it much safer.(21/24)

If case rates were higher (ie, >30/100K/d), testing wouldn’t reassure me enough to make me OK w/ being maskless in this size group. If they were much lower (ie, <10/100K/d), I’d be OK being maskless without the need for testing, as I’ve described. https://twitter.com/Bob_Wachter/status/1495165503106543618 (22/24)

I appreciate how confusing this all is, and it’s easy to see how the nuance can be lost – sometimes by well-meaning people who simply don’t understand the science, and sometimes by people who – for reasons I don’t profess to understand – want to… (23/24) https://twitter.com/EricTopol/status/1604552103959048192

... encourage their fellow human beings to put their health at risk. I hope this is helpful to you as you try to enjoy yourself and stay safe during this holiday season. (24/end)

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