Back-of-the-envelope Delta variant math

[Shermann]

I'm working off of this article:

https://www.cidrap.umn.edu/news-perspective/2021/06/cdc-delta-variant-now-10-us-covid-19-cases

So on May 22 the variant made up 2.7% of US cases (approx 750 out of 28,000).

Now the variant makes up 10% of cases (approx 1200 out of 12,400).

So despite the number of CV-19 cases going down, Delta cases are doubling every four weeks. If you think of the Delta variant as an independent pandemic, that gives it an R-naught value of around 1.4 (assuming a two week infectious period). That is significantly higher than the magic value of 1 that you must remain below for the pandemic to wane.

That's during a time where the vaccines are in effect, but vaccinations are trailing off. So in looking at it independently from other strains, it appears to be a pandemic within a pandemic that will continue to spread with exponential growth.

Please find a mistake in my math or reasoning.

[Scrivener7]

1. Right now the numbers in my state are fabulous. I'm getting out as much as I can. But

this thing bears watching like a hawk.

[AZSkiffyGeek]

2. But cases overall are down

So there are more Delta variants, but less overall. So while the percentage of Delta is increasing, it's because the total number of cases is dropping?
Or am I missing something (admittedly not a mathemetician).

[regnaD kciN]

3. There's still so much we don't know about this...

First of all, as you note, how much of Delta's growth is due to the other variants decreasing? Also, and just as crucially, how effective are the current vaccines against it? Studies in Europe apparently indicate the two vaccines in widespread use there (AstraZenica and, I believe, Moderna) are slightly less effective against it than against other variants, but nevertheless highly effective, but who knows about the other two vaccines?

[AZSkiffyGeek]

4. I've seen reports that the mRNA vaccines are effective

Not as much as against older variants, but still effective. I think I saw that a week or so after the CDC announcement that mask requirements weren't required for vaccinated people.

AstraZeneca doesn't seem to be effective enough, but that also isn't used here.

[OnDoutside]

10. Astrazeneca is 92% effective after the 2nd dose.

https://www.nbcnews.com/health/health-news/two-doses-vaccine-highly-effective-against-delta-variant-u-k-n1270776

Two doses of the Pfizer-BioNTech or the AstraZeneca Covid-19 vaccines are highly effective against hospitalization from the Delta variant of the coronavirus, according to a new analysis from Public Health England released Monday.

The variant, which was first identified in India, has become the predominant strain in the United Kingdom. A previous analysis from PHE suggested that a single dose of the vaccine was less effective against symptomatic illness caused by the Delta variant compared to the so-called Alpha variant, or B.1.1.7, which swept the U.K. in the winter.

Full coverage of the coronavirus outbreak

The new analysis found that two doses of the Pfizer vaccine were 96 percent effective against hospitalization from the Delta variant, and two doses of the AstraZeneca vaccine were 92 percent effective.

“The second shot is critical,” said Dr. Paul Offit, a vaccine researcher at the Children's Hospital of Philadelphia. “We know from the phase one studies that the second shot induces a level of virus-specific neutralizing antibodies that’s about tenfold greater than that after the first dose.”

[SoonerPride]

5. The Pfizer vaccine is 88% effective against the Delta variant

https://www.cnbc.com/2021/06/16/more-contagious-delta-covid-variant-could-become-dominant-in-us.html

The good news is that two doses of the currently available Covid vaccines in the U.S. appear to be effective against the delta strain.

A recent study out of the U.K. showed that the Pfizer-BioNTech mRNA vaccine is 88% effective against the delta variant two weeks after the second dose.

If you are vaccinated you don't really need to worry about it. Really.

[imaginary girl]

6. Any idea how J and J is?

I’ve been looking for efficacy numbers against Delta for J and J vaccine, but can’t find them. Have you seen any?

[SoonerPride]

7. I have not.

If I run across them I will post it for sure.

[imaginary girl]

17. Thanks! Nt

[Blues Heron]

8. from the figures in the OP, absolute numbers of Delta increased

Delta went from 750/day to 1200/day even as total Covid cases went down from 28000/day to 12,700/day

[ananda]

15. Pfizer is also fairly effective.

I think Pfizer and Moderna are said to be about 88%
effective againt Delta.

[Blues Heron]

9. absolute numbers of Delta went up even as total numbers fell

[Shermann]

11. Exactly

There is an alternative hypothesis to R-naught 1.4, however. That is that the R-naught of Delta is below 1 in this country, and any small outbreaks will burn out instead of spread. The observed increase in cases is due to fresh infusions of Delta virus from other countries where it is indeed growing exponentially.

That may be optimistic, however. The UK seems to be experiencing the beginning of an exponential resurgence.

So even if we are vaccinated, there may be lockdowns in our future due to the spread through the unvaccinated.

[BGBD]

19. n is too small to extrapolated

And increase of 500ish cases in a month doesn't tell you much about how it is actually spreading dland what it will do.

I find it unlikely that it can maintain a 1+ R0 in the US.

[Shermann]

20. Why unlikely?

The goal for vaccination has been 70% - 90%. Anything below that and we are at risk for spread. And we are below even the bottom end of that range. And yes, vaccinations are tapering off here.

[TraceNC]

12. What's (maybe) even worse...

The spread of the Delta variant is creating a growing pool of people who will serve as the breeding ground for variants on the Delta variant.

Already there’s evidence showing the Delta variant mutating in some other countries and not just changing one spike protein, but two. They’re calling this the "Lambda" variant. (Edit to add source.) https://www.cnbc.com/2021/06/16/who-says-delta-covid-variant-has-now-spread-to-80-countries-and-it-keeps-mutating.html There’s also some info there on the “Delta plus” variant.

So this group of unvaccinated people are placing all of us at risk of having to face a variant that the current vaccines can’t touch.

Then we’re back to Square One.

[Yavin4]

13. And yet, when I post that we should mandate vaccines, everyone clutches their Libertarian pearls.

Like when I post that we should have mandated that Medicaid and Medicare recipients get the vaccine, I was met with a hue and cry about how "it's not 100% approved" and "you cannot take people's Medicare away". AND YET, at the same time, some people are perfectly comfortable with locking us down again if the virus spins out of control. No problem forcing everyone to stay in their homes, but God forbid if we make people take a vaccine that could save their lives and end the virus.

If you're not in favor of mandating that people take the vaccine, you do not have the right to advocate for another lockdown.

[BannonsLiver]

14. We actually hit 2 million vaccinations today

So vaccines aren’t “tailing off”. I check the Bloomberg tracker every day and the average has been a steady 1 to 1.3 million a day for a while now. Yes, they are down relative to their highs in the early spring. But then again, so are cases.

How about we just wait and see how it goes rather than rooting for a specific outcome so we can be proven right?

[Yavin4]

16. A more plausible explanation is that the rate of transmission amongst the unvaxxed is spreading...

faster than the original strain, and at the same time, the number of fully vaxxed grows everyday. This explains why the overall case counts are falling but the Delta strain grows.

As long as there are large swaths of the American public unvaxxed, we will see the virus spread primarily among that population.

[Shermann]

18. This is entirely consistent with both of my hypotheses nt