Thread by @DrEricDing

HERD IMMUNITY THRESHOLD HARDER—Vaccine herd immunity is becoming harder. The formula is (1-(1/R0)) depends on virus contagiousness, which #B117 variant is 40-80% higher & will be dominant soon. So instead of 70% immune, we will now need 80-85%.🧵#COVID19
washingtonpost.com/health/covid-m…

2) “If a more transmissible strain becomes dominant, “that level of coverage needed for herd immunity would become higher, in the 80 to 85 percent range,” Jay Butler, deputy director for infectious diseases at CDC.”

3) Also hurting this target number is that the new South Africa 🇿🇦 #B1351 mutated variant (and likely the Brazil #P1 variant too because it shares the troublesome E484K mutation) is significantly dropping the protection offered by prior infection or from vaccines (many studies).

4) That’s why experts say “The latest bulletins about variants and the Novavax results in South Africa “really does make the prospect of herd immunity, at least before next winter, much less likely,” said Christopher Murray, director of the University of Washington’s @IHME_UW.

5) All three of the most-scrutinized “variants of concern” have arrived in the US. As of midday Saturday there were more than 430 reported cases involving 🇬🇧 variant #b117, and one case, in Minnesota of 🇧🇷 variant #P1 plus 🇿🇦 #B1351 found in SC and Maryland.

6) vaccines appear to work well against #B117 variant. But J&J vaccine did not appear to prevent disease quite as well in 🇿🇦 & LATAM — where problematic variants are spreading. And Novavax data showed a dramatic drop in effectiveness against the B.1.351 variant circulating in 🇿🇦.

7) “So far there is limited evidence of changes in disease severity from these variants. The exception is the variant spreading in the United Kingdom, which may be roughly 30 percent more lethal”

I reviewed the data. It’s concerning. See discussion 🧵 on the 10 studies ⬇️

https://twitter.com/DrEricDing/status/1353468238773035008

8) worrisome was the recent data that found blood from 48% of people previously recovered from #COVID19 in the 1st 2020 wave in South Africa did not recognize the new #B1351 variant — “complete immune escape”. 90% had lower neutralization.

https://twitter.com/DrEricDing/status/1351282325926146048

9) this is why South Africa’s CDC
@nicd_sa put out this stunning statement last week on the “reinfection” potential of the new #B1351 variant. Because of the 48% of recovered #COVID19 patients with “complete immune escape”

https://twitter.com/drericding/status/1351686170044989442

10) here is now the math of #B117 variant would compare with the old common variant... if there is just 10 cases now, there would be 2000+ by end of March—200x. But we know there are likely much more already now.

https://twitter.com/drericding/status/1354646619174678529

11) and it would be 6x difference in cases in just 2 weeks if there was a head to head race between the #B117 vs common old type.

https://twitter.com/drericding/status/1354648389712027654

13) also the herd immunity threshold of 80-85% includes children. About ~20% of the US is under 16 years of age. (Pfizer is approved for 16+ while Moderna approved for 18+). So basically we need 100% of adults vaccinated just to achieve 80% if no kids under 16 vaccinated yet.

14) That said, pediatric trials of the vaccines are underway already. They need to do more careful dosing studies for kids because kids age 1-15 vary in size considerably. China says its pediatric trials done by March. Western vaccine trials for kids likely late spring or summer.

15) That said, herd threshold can be lower if there is some natural immunity (Hope not too much since that is the dangerous approach & imperfect), and mitigation. Mitigation lowers the effective R. Basically need 80-85% immune if we did zero mitigation anymore (normal life).

16) We are mostly holding the effective R under 2 in most places, and under 1 in some places with masks, closures, etc. but every release valve we reopen, we raise the R and raise the effective % we need vaccinated again. It’s combined gas pedal + braking system we need to use.

17) but when I see this video... it makes me think mitigation is a lot harder than we think it is. More people vaccinated sooner is what we need if lockdowns aren’t enforced enough.

https://twitter.com/DrEricDing/status/1355718619490115587