Friday, April 03, 2020

Does California have "Herd Immunity?"

Recently, reports of a study from Sunetra Gupta, professor of theoretical epidemiology at Oxford University, "went viral," claiming that the United Kingdom might not suffer so badly from Covid-19 because it already had achieved something called "herd immunity."  The virus had already made the rounds and rendered enough people immune to it that it couldn't spread far or fast, like trying to start a wet log on fire.  

Herd Immunity AND luck of the Irish!
On Monday, Victor Davis Hanson offered the same suggestion about California at the end of an otherwise interesting article on National Review. Then Rush Limbaugh, saying he likes to think for himself, pushed this theory today on his show. This theory has now percolated down to the popular level, for instance this heartfelt and troubling story from an intelligent Arizona Mom who has been (stupidly) denied cancer treatment because of the pandemic, which she thinks has already passed. Another article claiming Covid-19 was "on its way out" in Oregon, and New York, so shutting the states down destroyed their economies for nothing, has gone viral. The premise also seems to be that the good people of Oregon State must have also cycled through this disease and come out the far end -- without having the bad manners of dying of it in mass, as elsewhere.
Why are there far fewer Covid-19 cases in California than New York? The theory is that Californians were actually hit by the virus much earlier, and gained "herd immunity." Californians are now sharing stories on Facebook of how they felt sick earlier this year and got over it.

Herd immunity is real, and easy enough to understand in theory. One can compare it to wearing masks.

Suppose you have a disease that spreads from one sick person on average to two others. Those two in turn cough on four others, then eight, then sixteen. After five generations of illness, you have 31 sick people, supposing none have recovered or passed away yet.

Now suppose everyone wears masks that prevents one half of all transmissions. Or suppose half the people have already been exposed to the disease and developed effective antibodies.

Now one person coughs on two, but only one catches the disease. And she coughs on two others, but only one gets ill. So instead of having 31 sick people by the fifth generation, you only have five. Add another five generations, and the ratio of sick people in these two hypotheses is 100-1. You have an epidemic in one place, but a minor outbreak that health officials can deal with between trips to the back nine, in the other.

We should not apply the Herd Immunity theory to California, I will argue. Far better to rely on masks.

Sunetra Gupta on British Herd Immunity

Sunetra Gupta, a theoretical epidemiologist at Oxford University, expressed surprise that the "Imperial Model" warning of possible high loss of life in the UK and US from Covid-19 (up to 2.2 million in the United States, if nothing was done), had been so easily accepted and acted upon.  According to the Financial Times, her own team found reason for optimism in the idea that both countries had also developed "herd immunity:"  

"Professor Gupta led a team of researchers at Oxford in a modeling study which suggests that the virus has been invisibly spreading for at least a month earlier than suspected, concluding that as many as half of the people in the United Kingdom have already been infected by COVID-19."
If that is so, hardly anyone who gets the disease gets really sick.  Less that one in a thousand will even need to stay in the hospital.  Good news!  The dreaded pandemic had already largely washed through Britain with hardly a ripple.  

Victor Davis Hanson on California Herd Immunity
Victor Hanson, an often insightful historian and curmudgeon, then asked on March 31 why New York and not California has been struck so hard by the disease.  Twenty times as many New Yorkers had died, as of his writing, than Californians, per capita.  

Citing Gupta, Hanson suggested that maybe the reason things aren't so bad yet in California, is residents had already gone through this wave: 

"Many Californians complained late in 2019 of getting the flu a bit early, with flu symptoms that were somewhat different from the norm, at times including severe muscle aches, some digestive cramping, an unproductive cough, and days or even weeks of post-fever fatigue."

Suppose that was Covid-19 in disguise? 

"So given the state’s unprecedented direct air access to China, and given its large expatriate and tourist Chinese communities, especially in its huge denser metropolitan corridors in Los Angeles and the Bay Area, it could be that what thousands of Californians experienced as an unusually “early” and “bad” flu season might have also reflected an early coronavirus epidemic, suggesting that many more Californians per capita than in other states may have acquired immunity to the virus."

Sorry to disappoint you Californians and Brits. But I think this "Herd Immunity" theory is historically unsound as it applies to both places, for five reasons:

(1) It is suspicious that Hanson's hypothesis purports to explain both why Covid-19 has NOT exploded in California, and why it HAS exploded in Lombardy. (Which, he says, also experienced unfamiliar respiratory problems early in winter.) When your theory is that flexible, you should step back and put on your N95 BS protection mask on, so as not to get sprayed by particles.

(2) Where is the positive evidence that millions of Brits or Californians contracted Covid-19? A few anecdotal coughs and fevers, such as one does get in winter? Sorry, that's not enough. We also need to assume the public health professionals and most of the doctors and scientists were easily fooled, while an admittedly brilliant historian and a clever radio talk show host see through the fog. (Gupta is an expert in a more relevant field. But still, she is a theoretical epidemiologist. What her theory most lacks is rootedness to facts, as we shall see.) The theory is speculative, mildly conspiratorial (because it assumes most of the experts are out to lunch or too hidebound to add two and two), and runs the danger of wishful thinking.
(3) Covid-19 has caused massive fatalities where even a large minority of people are known to be infected, such as Lombardy, the Barcelona and Madrid areas, Iran, Holland, and eastern France. Known deaths from this disease in France yesterday and today were some 2400, not counting those outside of hospitals, as the Wall Street Journal reported yesterday. And even in regions where the disease has spread most virulently, most people have tested negative: only some 35% tested positive even in Lombardy, Italy.

That's why most of the population is still hiding indoors.

Gupta's theory that only one in a thousand infected people even needs to go the hospital is refuted by Lombardy alone, where even with only 40% of those tested showing positive, and only a small fraction of the population tested, one in a thousand citizens has not only stayed in the hospital, but died of the disease in that region. Gupta's speculative numbers have already been falsified in the harshest way possible.

And the disease has proven little more gentle in Spain, Holland, Sweden, or now New York or London. This disease doesn't come unheralded or leave unmourned.
(4) Testing on Covid-19 in California has shown a strong rise over the past month or so. After 7,981 had been tested by March 16th (presumably testing greatly favored those who showed symptoms or who had been exposed), only 335 people tested positive, a bit more than 4%. By contrast, almost half of those tested in the past week, some 14,000 (aside from the many tests still out), have tested positive.

(5) Geneticists can trace the history of Covid-19 through its mutations, and that history does not seem to support the "Early Immigrant Herd Immunity" theory. (6) The Seattle Flu Study seems to put the final nail in the coffin of this theory. (And more recently, antibody tests in California.)

So sorry, California Dreamers. (And Brits.) Hunker down. The crest of the wave is still coming.

But that doesn't mean California has to experience what New York did.  

And why is that?  

Why did Covid-19 explode in New York, but not California?  

So why did Covid-19 spread so rapidly in New York, but less rapidly in California, or here in Washington, where it got the earliest start of all? 

Earlier, I analyzed nine hypotheses for why this disease has spread rapidly in some places, but not others.  (I have now revised that post from two weeks ago.)  I called these "A Culture of Contact," "Bad Faith," "Cool, Slightly Drier Weather," "The Camel Cries" (travel), "A Human Petri Dish," "Socialized Medicine," "The Donald," "Cultural Arrogance," and "The Judgement of God."  

See the source image
* Culture of Contact surely must be a large part of the answer.  New York City, like Wuhan, Milan, and Barcelona, are all huge cities with well-developed subway systems.  New Yorkers are "packed on top of one another."  The New York subway network sees 1.6 billion riders a year, seven times the nation's second largest system in Chicago.  By contrast, people in Los Angeles pretty much all drive, with the local trains gathering only 42 million riders a year.  While San Francisco is one of America's most crowded cities, BART receives only 1/12th of the ridership of New York's subways. 

Before the outbreak, a study showed that the New York subway was already a stew of bacteria and viruses.  Furthermore, the outbreak in New York generally seems to follow commuting patterns, into New Jersey and other suburban areas.   

* Southern California's desert-like weather might have impeded the spread of the disease in two ways: by killing the virus when broadcast out-of-doors more quickly, and by keeping people out of doors.  (Sitting on lawn chairs at Cal Tech is feasible in February, lying on the grass at Columbia less inviting that early.  Cough into the wind, and most virus diffuse and die.)  

* It is true that California (and Washington) host many flights from East Asia.  But so does New York, also from Europe, so that factor is probably more-or-less a wash.  

* Arrogance, human sin generally, the authority of Donald Trump, and medical systems, are no doubt relatively constant between the two population centers east and west.  

* Another factor which Limbaugh mentioned was that California closed down two days earlier than New York.  He scoffed at the idea that two days could matter.  But of course, a lot can happen in two days. 

My impression is, Washington State fought its earliest-in-the-nation outbreak more aggressively and professionally than some other states.  The concentration of tech fire-power here and institutions like the University of Washington Med School (and Virology Department), Fred Hutchinson Cancer Center, and the Bill and Melinda Gates Foundation, have it seems to me helped push back against the spread of Covid-19 in a more unified and forceful way than some states may have managed.  Washington has conducted more tests than almost any other state.  Also, like Boston, Seattle has a highly-educated population, who likely take better care of themselves. 

Some of these factors likely also play a role in the Bay Area, though less in Los Angeles, where California cases are concentrated.  

* Neither should one completely discount the importance of dumb luck.   Once a disease is wide-spread, the numbers are likely to even out.  But as Washington State shows, one unfortunate traveler who coughs in the wrong place can lead to a major outbreak a month later.  (His case history and its aftermath has been carefully studied.)  

So no, I don't think "Herd Immunity" helps explain why California has gotten off relatively easily so far, at least compared to it's East Coast rival.  Neither is this theory needed.  Population density, commuting patterns, climate , and bad luck, do more, I think, to explain why New York and not Los Angeles, San Francisco, or Seattle, have been most terribly struck by the Covid-19 epidemic to date.

Testing has been lax in California, however, testing the state's better luck.   While Washington State has tested 80,000 people, California, with more than five times the population, still only has results for 35,000 (with 60,000 more results waiting to come in.)  Probably many more now have the disease than have been verified as having it. 

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