If you’ve been reading about Covid-19 in the New York Times, you’ve probably learned that reinfection is very unlikely.
What you’ve learned is incorrect.
Don’t get me wrong – I love the New York Times. Within the spectrum of United States politics, I am very far to the left. Anti-consumerist, prison abolitionist, environmentalist, feminist, climate activist, etc., etc. I fit into all those categories.
I’m also a scientist. I am staunchly pro-vaccine. I don’t like pesticides, but I’m a huge fan of GMO crops. (Honestly, I wish there was a category at the grocery store where you could pay to support genetically-modified organisms grown without environmental toxins – “organic” doesn’t have the nuance I’d like.)
So my goal here isn’t to rag on the New York Times. I’m including screenshots of their headlines only to give us a common frame of reference.
This is what the news is saying. And it’s wrong.
It was going to be very difficult to demonstrate reinfection with Covid-19.
In general, reinfection with any virus will produce a milder illness the second time.
Most people’s first infection with Covid-19 is so mild that they don’t realize they have it – perhaps 80% of infections are “asymptomatic,” in which a person has been infected with the virus, is probably shedding the virus (thereby infecting other people), but feels totally fine. So, people’s second infection? Some percentage higher than 80% are likely to feel totally well, even though they might be shedding virus.
When people develop severe complications from Covid-19, the illness can linger for weeks or even months.
I don’t know for certain whether my family contracted Covid-19 in February, because there were no tests available here at the time. All I know is that we were two close contacts removed from someone who had just returned from China, that this close contact tested negative for influenza, that my family had been vaccinated for influenza, and that our symptoms precisely mirrored the common suite for Covid-19. But in any case, we felt horrible for about three weeks, and we experienced lingering fatigue with occasional coughing for about two months.
Lengthy recovery is so common that there’s a colloquial name for it: “long-haulers.” If we’re trying to identify whether someone was re-infected, we’d need to make sure that we weren’t looking at continued viral shedding during a lengthy recovery.
To demonstrate that someone was re-infected with Covid-19, the following would have to happen:
A person gets tested for Covid-19 during their first infection.
The genome of the virus is sequenced after that first infection.
The person is re-infected.
The person happens to get a Covid-19 test during the second infection (even though it’s highly likely that this person feels well at the time).
The genome of the virus is sequenced after the second infection.
The genome of the virus that infected the person on the second occasion is noticeably different from the first (even though Covid-19 includes a proofreading enzyme that slows genetic drift).
That’s all very unlikely!
There are just so many coincidences involved – that you happen to get infected with an easily distinguishable virus the second time, that you happen to get a test the second time, that anyone took the (significant) trouble and expense to sequence both genomes.
And what I mean is, proving re-infection is very unlikely. Which is totally independent of the likelihood of re-infection itself.
And yet, even though it’s so unlikely we’d be able to prove that re-infection is occurring, we have.
Given how unlikely it was that we’d be able to document reinfection, the fact that we’ve seen this at all indicates that it’s probably quite common. As you would expect based upon our bodies’ responses to other coronaviruses.
Given that re-infection definitely occurs, and is probably quite common, why have you read that it’s unlikely?
The underlying probably is language usage. When my father – an infectious diseases specialist – talks about re-infection, he’s thinking about contracting severe symptoms during a second infection. Which is reasonable. He’s a medical doctor. He cares about helping sick people get better.
But when we’re thinking about how to respond, as a nation, to this pandemic, we’re thinking about the dynamics of transmission. We’re trying to answer questions like, “Can kids go to school without people dying?”
From this perspective, we’re thinking about who is going to spread the virus, and where. We need to know whether a person who is protected from severe disease – either from prior recovery or vaccination – might shed viral particles. Will that individual register as a positive case on a PCR test? Will that individual get classmates or co-workers sick?
Re-infections are probably the underlying cause of the current rise in cases in New York City.
70% or more of the population of New York City was infected with Covid-19 during April. That’s a huge percentage, well above what most researchers consider the “herd immunity threshold” for similar respiratory viruses.
For there to be another spike in cases now, many of those 70% would need to have lost their initial immunity. That’s also why you’d expect to see a higher “test positivity rate” – if many of the current cases are reinfections, then they’re likely to be milder. People with milder (or asymptomatic) infections are less likely to seek out a test.
For general audiences, the phrasing I’d recommend is to say “Severe illness is unlikely during Covid-19 reinfections” as opposed to “Reinfection is unlikely.”
There have been a few cases of people’s second infection being more severe than the first, but these cases indeed appear to be quite rare.
But re-infection itself?
The fact that we’ve documented any instances of re-infection suggests that it’s quite common. Which we could have predicted from the beginning – indeed, I did. And that’s why I’ve been recommending – for months – policies very different from what we’ve done.
We’re fast approaching flu season, which is especially harrowing this year.
We, as a people, have struggled to respond to this calamity. We have a lot of scientific data about Covid-19 now, but science is never value-neutral. The way we design experiments reflects our biases; the way we report our findings, even more so.
For example, many people know the history of Edward Jenner inventing the world’s first vaccine. Fewer are aware of the long history of inoculation in Africa (essentially, low-tech vaccination) that preceded Jenner’s work.
So it’s worthwhile taking a moment to consider the current data on Covid-19.
Data alone can’t tell us what to do – the course of action we choose will reflect our values as a society. But the data may surprise a lot of people – which is strange considering how much we all feel that we know about Covid-19.
Indeed, we may realize that our response so far goes against our professed values.
Spoiler: I think we shouldn’t close in-person school.
Since April, I’ve written severalessays about Covid-19. In these, I’ve made a number of predictions. It’s worthwhile to consider how accurate these predictions have been.
This, after all, is what science is. We use data to make an informed prediction, and then we collect more data to evaluate how good our prediction was.
Without the second step – a reckoning with our success or failure – we’re just slinging bullshit.
I predicted that our PCR tests were missing most Covid-19 infections, that people’s immunity was likely to be short-lived (lasting for months, not years), and that Covid-19 was less dangerous than seasonal influenza for young people.
In my essays, I’ve tried to unpack the implications of each of these. From the vantage of the present, with much more data at our disposal, I still stand by what I’ve written.
But gloating’s no fun. So I’d rather start with what I got wrong.
My initial predictions about Covid-19 were terrible.
I didn’t articulate my beliefs at the time, but they can be inferred from my actions. In December, January, and February, I made absolutely no changes to my usual life. I didn’t recommend that travelers be quarantined. I didn’t care enough to even follow the news, aside from a cursory glance at the headlines.
While volunteering with the high school running team, I was jogging with a young man who was finishing up his EMT training.
“That new coronavirus is really scary,” he said. “There’s no immunity, and there’s no cure for it.”
I shrugged. I didn’t know anything about the new coronavirus. I talked with him about the 1918 influenza epidemic instead.
I didn’t make any change in my life until mid-March. And even then, what did I do?
I called my brother and talked to him about the pizza restaurant – he needed a plan in case there was no in-person dining for a few months.
My next set of predictions were off, but in the other direction – I estimated that Covid-19 was about four-fold more dangerous than seasonal influenza. The current best estimate from the CDC is that Covid-19 is about twice as dangerous, with an infection fatality ratio of 0.25%.
But seasonal influenza typically infects a tenth of our population, or less.
We’re unlikely to see a significant disruption in the transmission of Covid-19 (this is the concept of “herd immunity”) until about 50% of our population has immunity from it, whether from vaccination or recovery. Or possibly higher – in some densely populated areas, Covid-19 has spread until 70% (in NYC) or even 90% (in prisons) of people have contracted the disease.
Population density is hugely important for the dynamics of Covid-19’s spread, so it’s difficult to predict a nation-wide threshold for herd immunity. For a ballpark estimate, we could calculate what we’d see with a herd immunity threshold of about 40% in rural areas and 60% in urban areas.
Plugging in some numbers, 330 million people, 80% urban population, 0.25% IFR, 60% herd immunity threshold in urban areas, we’d anticipate 450,000 deaths.
That’s about half of what I predicted. And you know what? That’s awful.
Each of those 450,000 is a person. Someone with friends and family. And “slow the spread” doesn’t help them, it just stretches our grieving to encompass a whole year of tragedy instead of a horrific month of tragedy.
Based on the initial data, I concluded that the age demographics for Covid-19 risk were skewed more heavily toward elderly people than influenza risk.
I may have been wrong.
It’s difficult to directly compare the dangers of influenza to the dangers of Covid-19. Both are deadly diseases. Both result in hospitalizations and death. Both are more dangerous for elderly or immunocompromised people, but both also kill young, healthy people.
Typically, we use an antigen test for influenza and a PCR-based test for Covid-19. The PCR test is significantly more sensitive, so it’s easier to determine whether Covid-19 is involved a person’s death. If there are any viral particles in a sample, PCR will detect them. Whereas antigen tests have a much higher “false negative” rate.
Instead of using data from these tests, I looked at the total set of pneumonia deaths. Many different viruses can cause pneumonia symptoms, but the biggest culprits are influenza and, in 2020, Covid-19.
So I used these data to ask a simple question – in 2020, are the people dying of pneumonia disproportionately more elderly than in other years?
I expected that they would be. That is, after all, the prediction from my claims about Covid-19 demographic risks.
For people under the age of 18, we’ve seen the same number of deaths (or fewer) in 2020 as in other years. The introduction of Covid-19 appears to have caused no increased risk for these people.
But for people of all other ages, there have been almost three times as many people dying of these symptoms in 2020 compared to other years.
In most years, one thousand people aged 25-34 die of these symptoms; in 2020, three thousand have died. In most years, two thousand people aged 35-44 die of these symptoms; in 2020, six thousand have died. This same ratio holds for all ages above eighteen.
Younger people are at much less risk of harm from Covid-19 than older people are. But, aside from children under the age of eighteen, they don’t seem to be exceptionally protected.
Of course, my predictions about the age skew of risk might be less incorrect than I’m claiming here. If people’s dramatically altered behavior in 2020 has changed the demographics of exposure as compared to other years – which is what we should be doing to save the most lives – then we could see numbers like this even if Covid-19 had the risk skew that I initially predicted.
I predicted that four or more years would pass before we’d be able to vaccinate significant numbers of people against Covid-19.
I sure hope that I was wrong!
We now know that it should be relatively easy to confer immunity to Covid-19. Infection with other coronaviruses, including those that cause common colds, induce the production of protective antibodies. This may partly explain the low risk for children – because they get exposed to common-cold-causing coronaviruses so often, they may have high levels of protective antibodies all the time.
Several pharmaceutical companies have reported great results for their vaccine trials. Protection rates over 90%.
So the problem facing us now is manufacturing and distributing enough doses. But, honestly, that’s the sort of engineering problem that can easily be addressed by throwing money at it. Totally unlike the problem with HIV vaccines, which is that the basic science isn’t there – we just don’t know how to make a vaccine against HIV. No amount of money thrown at that problem would guarantee wide distribution of an effective vaccine.
We will still have to overcome the (unfortunately significant) hurdle of convincing people to be vaccinated.
For any individual, the risk of Covid-19 is about twice the risk of seasonal influenza. But huge numbers of people choose not to get a flu vaccine each year. In the past, the United States has had a vaccination rate of about 50%. Here’s hoping that this year will be different.
Which means that elderly people will always be at risk of dying from Covid-19.
The only way to protect people whose bodies have gone through “age-related immunosenence” – the inevitable weakening of an immune system after a person passes the evolutionarily-determined natural human lifespan of about 75 years – will be to vaccinate everybody else.
Depending on how long vaccine-conferred immunity lasts, we may need to vaccinate people annually. I worry, though, that it will become increasingly difficult to persuade people to get a Covid-19 vaccine once the yearly death toll drops to influenza-like levels – 50,000 to 100,000 deaths per year in the United States.
(Note: you may have seen articles in the New York Times suggesting that we’ll have long-lasting protection. They’re addressing a different question — after recovery, or vaccination, are you likely to become severely ill with Covid-19? And the answer is “probably not,” although it’s possible. When I discuss immunity here, I mean “after recovery, or vaccination, are you likely to be able to spread the virus after re-infection?” And the answer is almost certainly “yes, within months.”)
And I wrote about the interplay between short-lived immunity and the transmission dynamics of an extremely virulent, air-born virus.
This is what the Harvard public health team got so wrong. When we slow transmission enough that a virus is still circulating after people’s immunity wanes, they can get sick again.
For this person, the consequences aren’t so dire – an individual is likely to get less sick with each subsequent infection by a virus. But the implications for those who have not yet been exposed are horrible. The virus circulates forever, and people with naive immune systems are always in danger.
It’s the same dynamics as when European voyagers traveled to the Americas. Because the European people’s ancestors lived in unsanitary conditions surrounded by farm animals, they’d cultivated a whole host of zoogenic pathogens (like influenza and this new coronavirus). The Europeans got sick from these viruses often – they’d cough and sneeze, have a runny nose, some inflammation, a headache.
In the Americas, there were fewer endemic diseases. Year by year, people wouldn’t spend much time sick. Which sounds great, honestly – I would love to go a whole year without headaches.
But then the disgusting Europeans reached the Americas. The Europeans coughed and sneezed. The Americans died.
And then the Europeans set about murdering anyone who recovered. Today, descendants of the few survivors are made to feel like second-class citizens in their ancestral homelands.
In a world with endemic diseases, people who have never been exposed will always be at risk.
That’s why predictions made in venues such as the August New York Times editorial claiming that a six- to eight-week lockdown would stop Covid-19 were so clearly false. They wrote:
Six to eight weeks. That’s how long some of the nation’s leading public health experts say it would take to finally get the United States’ coronavirus epidemic under control.
For proof, look at Germany. Or Thailand. Or France.
Obviously, this didn’t work – in the presence of an endemic pathogen, the lockdowns preserved a large pool of people with naive immune systems, and they allowed enough time to pass that people who’d been sick lost their initial immunity. After a few months of seeming calm, case numbers rose again. For proof, look at Germany. Or France.
Case numbers are currently low in Thailand, but a new outbreak could be seeded at any time.
And the same thing is currently happening in NYC. Seven months after the initial outbreak, immunity has waned; case numbers are rising; people with mild second infections might be spreading the virus to friends or neighbors who weren’t infected previously.
All of which is why I initially thought that universal mask orders were a bad idea.
We’ve known for over a hundred years that masks would slow the spread of a virus. The only question was whether slowing the spread of Covid-19 would cause more people to die of Covid-19.
And it would – if a vaccine was years away.
But we may have vaccines within a year. Which means that I may have been wrong. Again, the dynamics of Covid-19 transmission are still poorly understood – I’ll try to explain some of this below.
In any case, I’ve always complied with our mask orders. I wear a mask – in stores, at school pickup, any time I pass within six feet of people while jogging.
To address global problems like Covid-19 and climate change, we need global consensus. One renegade polluting wantonly, or spewing viral particles into the air, could endanger the whole world. This is precisely the sort of circumstance where personal freedom is less important than community consensus.
The transmission dynamics of Covid-19 are extremely sensitive to environment. Whether you’re indoors or outdoors. How fast the air is moving. The population density. How close people are standing. Whether they’re wearing masks. Whether they’re shouting or speaking quietly.
Because there are so many variable, we don’t have good data. My father attended a lecture and a colleague (whom he admires) said, “Covid-19 is three-fold more infectious than seasonal influenza.” Which is bullshit – the transmission dynamics are different, so the relative infectivity depends on our behaviors. You can’t make a claim like this.
It’s difficult to measure precisely how well masks are slowing the spread of this virus.
But here’s a good estimate: according to Hsiang et al., the number of cases of Covid-19, left unchecked, might have increased exponentially at a rate of about 34% per day in the United States.
That’s fast. If about 1% of the population was infected, it could spread to everyone within a week or two. In NYC, Covid-19 appear to spread to over 70% of the population within about a month.
(To estimate the number of infections in New York City, I’m looking at the number of people who died and dividing by 0.004 – this is much higher than the infection fatality rate eventually reported by the CDC, but early in the epidemic, we were treating people with hydroxychloraquine, an unhelpful poison, and rushing to put people on ventilators. We now know that ventilation is so dangerous that it should only be used as a last resort, and that a much more effective therapy is to ask people to lie on their stomachs – “proning” makes it easier to get enough oxygen even when the virus has weakened a person’s lungs.)
Masks dramatically slow the rate of transmission.
A study conducted at a military college – where full-time mask-wearing and social distancing were strictly enforced – showed that the number of cases increased from 1% to 3% of the population over the course of two weeks.
So, some math! Solve by taking ten to the power of (log 3)/14, which gives an exponential growth rate of 8% per day. Five-fold slower than without masks.
But 8% per day is still fast.
Even though we might be able to vaccinate large numbers of people by the end of next year, that’s not soon enough. Most of us will have been sick with this – at least once – before then.
I don’t mean to sound like a broken record, but the biggest benefit of wearing masks isn’t that we slow the rate of spread for everyone — exponential growth of 8% is still fast — but that we’re better able to protect the people who need to be protected. Covid-19 is deadly, and we really don’t want high-risk people to be infected with it.
I’ve tried to walk you through the reasoning here — the actual science behind mask policies — but also, in case it wasn’t absolutely clear: please comply with your local mask policy.
You should wear a mask around people who aren’t in your (small) network of close contacts.
I’m writing this essay the day after New York City announced the end of in-person classes for school children.
A major problem with our response to Covid-19 is that there’s a time lag between our actions and the consequences. Human brains are bad at understanding laggy data. It’s not our fault. Our ancestors lived in a world where they’d throw a spear at an antelope, see the antelope die, and then eat it. Immediate cause and effect makes intuitive sense.
Delayed cause and effect is tricky.
If somebody hosts a party, there might be an increase in the number of people who get sick in the community over the next three weeks. Which causes an increase in the number of hospitalizations about two weeks after that. Which causes people to die about three weeks after that.
There’s a two-month gap between the party and the death. The connection is difficult for our brains to grasp.
As a direct consequence, we’ve got ass-hats and hypocrites attending parties for, say, their newly appointed Supreme Court justice.
But the problem with school closures is worse. There’s a thirty year gap between the school closure and the death. The connection is even more difficult to spot.
The authors link two sets of existing data: the correlation between school closures and low educational achievement, and the correlation between low educational achievement and premature death.
The public debate has pitted “school closures” against “lives saved,” or the education of children against the health of the community. Presenting the tradeoffs in this way obscures the very real health consequences of interrupted education.
These consequences are especially dire for young children.
The authors calculate that elementary school closures in the United States might have (already!) caused 5.5 million years of life lost.
Hsiang et al. found that school closures probably gave us no benefit in terms of reducing the number of Covid-19 cases, because children under 18 aren’t significant vectors for transmission (elementary-aged children even less so), but even if school closures had reduced the number of Covid-19 cases, closing schools would have caused more total years of life to be lost than saved.
The problem – from a political standpoint – is that Covid-19 kills older people, who vote, whereas school closures kill young people, who are intentionally disenfranchised.
And, personally, as someone with far-left political views, it’s sickening for me to see “my” political party adopt policies that are so destructive to children and disadvantaged people.
So, here’s what the scientific data can tell us so far:
We will eventually have effective vaccines for Covid-19. Probably within a year.
Covid-19 spreads even with social distancing and masks, but the spread is slower.
You have no way of knowing the risk status of people in a stranger’s bubble. (Please, follow your local mask orders!)
Schools – especially elementary schools – don’t contribute much to the spread of Covid-19.
School closures shorten children’s lives (and that’s not even accounting for their quality of life over the coming decades).
An individual case of Covid-19 is about twice as dangerous as a case of seasonal influenza (which is scary!).
Underlying immunity (from prior disease and vaccination) to Covid-19 is much lower than for seasonal influenza, so there will be many more cases.
Most people’s immunity to Covid-19 probably lasts several months, after which a person can be re-infected and spread the virus again.
So, those are some data. But data don’t tell us what to do. Only our values can do that.