An alternative explanation is that of "viral interference"[1,2], i.e., the most transmissible virus boosting up viral immunity in the population, and precluding transmission of the less transmissible/fit virus
The first, "alternative explanation" the statistician in me wants to mention is perhaps the way influenza data is gathered has changed.
"Only people who get tested for influenzalike illnesses—typically about 5 percent of individuals who fall ill—are tallied."
If there were a change in the way these tests were administered, for example a blaring medical bias toward another disease, that would present a significant sampling problem. That same statistician also wanted me to mention the simplest and most boring answers are usually closest to the truth.
A bias towards testing for Covid wouldn't explain fewer cases of the flu unless those cases of the flu were coming back as Covid falsely instead of so further diagnosis was stopped. If you have a bad enough flue to seek treatment, then once you get that Covid test and it says "negative", you would move on to the next step for treatment. (Consistent with past years - nobody was getting flu tested just cause they had a runny nose in 2019, it was just the people who needed treatment).
There could be other explanations, such as: a desire to avoid Covid causes people to avoid seeking treatment, so more flu cases self resolve. OR: a fear of Covid causes more people to get tested when they're sick, and some of them may then go for flu testing before they would otherwise after coming back negative for Covid...?
Seems like the simplest answer is just "actions that have reduced the spread of one disease have also reduced the spread of another, that's historically less widespread already."
> Seems like the simplest answer is just "actions that have reduced the spread of one disease have also reduced the spread of another, that's historically less widespread already."
This is not the simplest answer, and the evidence that the measures have really slowed spread is extremely low except in places like New Zealand and Australia which are small islands in Oceania.
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> A bias towards testing for Covid wouldn't explain fewer cases of the flu unless those cases of the flu were coming back as Covid falsely instead of so further diagnosis was stopped.
Exactly this. I'm partial to the viral interference hypothesis, but what you don't seem to realize is that if you get infected for SARS-CoV-2 and recover in 7-14 days, you will still test PCR+ for months after. This goes into the widespread mistuning of the cycle threshold. Case in point: They tested George Floyd's corpse for COVID-19 and he was PCR+, despite having recovered from COVID-19 a couple months before. The test hit on the remnant viral debris from his long-gone infection.
> This is not the simplest answer, and the evidence that the measures have really slowed spread is extremely low except in places like New Zealand and Australia which are small islands in Oceania.
Have you ever heard of this small Asian country called CHINA?
With a lot of people getting tested for COVID "just in case" and hypochondriacs constantly checking in with their doctors, I'd guess that the percent of flu cases tallied might actually be a little bit higher than usual due to COVID
I wouldn't be surprised if there was more flu testing this year. I've never intentionally taking a flu test, but some of my COVID tests also screened for influenza a/b at the same time.
The lack of Flu infections in the population that did not contract COVID cannot be explained by viral interference. Viral interference would only explain a decreased influenza infection rate among those exposed to COVID.
If a large portion of the people who contracted COVID are removed from the set of people who could have potentially exposed me to the flu, doesn't that decrease the likelihood of me contracting the flu regardless of whether I contracted COVID?
Removing those people removes, relatively, an extremely small portion of the population. You are removing roughly 10% of the population in the U.S., or 2% worldwide. This does not explain the magnitude of the decrease observed for the flu.
Is it that hard to believe that not being around other people limits the chance of getting sick from other people?
Combination between efforts to avoid COVID and viral interference of those who were exposed would drastically reduce the ability to spread. Could it perhaps have reduced it enough to have stopped a flu season from developing?
That's pretty much my belief: efforts to avoid COVID also avoid the flu. Viral interference, theoretically, is part of that picture, but does not fully explain the observed data and has not, to my knowledge been tested itself: references to the phenomena as an explanation for flu decreases are based on research for other viruses. Given the HN community frequently criticizes research or reporting on research for overstating it's claims, I am surprised that so many people here have latched on to an extrapolation from research not directly related to COVID.
It seems far simpler, in the Occam's Razor sense, to begin with the idea that staying away from people makes it harder to get sick from other people.
Unfortunately, COVID restrictions and social distancing have become so politicized that many resist the idea that staying away from people reduces the chance of getting the flu from them, because the implication is that those measures might also have been effective in reducing COVID cases. This contradicts a political point of view and therefore some would like to dismiss it out if hand.
For example I've notice that many of those who believe precautions violated their personal liberty want to diminish the effectiveness of those precautions, which isn't necessary from a logical point of view. It is not logically inconsistent to believe these precautions violated civil liberties even if they were highly effective. It just requires a person to believe that many deaths were a reasonable price to pay.
Those links don't allege what you are saying at all... the second expressly disclaims any such conclusions given the size of the error bars, and the first talks about the protective effect of an active infection, not a simultaneous epidemic. Needless to say, only a tiny fraction of humanity has been actually infected with covid at any one time over the past year.
Do you have a link to someone alleging this for covid? I'm worried this is spin that you picked up from a source that has an interest in opposition to covid mitigation practices.
I thought exactly the same. I recently had a lengthy discussion about viral interference here on HN only to learn that the person didn't want to see evidence that the mitigation measures (i.e. associated human behaviour change!) are effective - not only for SARS-Cov-2, but for a broad range of viral (and non-viral!) infectious diseases.
Not necessarily. COVID might be so transmissible that the entire world has been exposed to it enough to trigger an immune response but only establishes itself and becomes an infection in those who get a large enough viral load over a short enough time period.
It wouldn't produce an immunity, it would just keep the innate immune response active enough to prevent the average small cluster of COVID or flu viruses from replicating and growing into a full blown infection. This wouldn't even give the adaptive immune system a chance to develop an immunity to either of them.
If someone went to the COVID or flu ward and had someone cough in their face, they'd still get infected because that kind of exposure is generally too much for the innate immune system to catch.
It's strange to call this an "alternative explanation", since it's the one that expert infectious disease researchers, nearly without exception, are pointing to.
The (let's face it, silly) notion that masks and distancing suppressed all respiratory pathogens except one is a theory with very little traction, and which will almost certainly not stand up to serious scrutiny.
> The (let's face it, silly) notion that masks and distancing suppressed all respiratory pathogens except one is a theory with very little tracti
Are you familiar with the concept of "R0" and that one virus may be more transmissible than another?
Flu has an R0 of about 1.5, and whatever covid's is, it's higher (last I saw was 2.5).
If we postulate that masking and distancing and similar measures act to reduce these numbers across the board, say by 0.6, that brings flu down to 0.9 and covid to 1.9. Outbreaks don't tend to stay around when their R0 is below 1, which would explain why there's hardly any flu this year and plenty of covid.
In this admittedly over-simplified model, beta is (per wiki) "the average number of contacts per person per time, multiplied by the probability of disease transmission in a contact between a susceptible and an infectious subject".
Social distancing reduces the average number of contacts, masking reduces the probability of transmission, and both act to reduce beta. You reduce beta, you reduce R0, you win.
There’s nothing silly about a novel virus having greater abilities to overcome barriers when compared to other pathogens that humans have had years decades and centuries to develop defenses again.
At the end of the day it’s likely gonna be the result of a combination of all these factors.
> There’s nothing silly about a novel virus having greater abilities to overcome barriers when compared to other pathogens that humans have had years decades and centuries to develop defenses again.
This isn't really quite accurate though. Before SARS-CoV-2 ever emerged, our immune systems already had defenses against it. This is because of the already-extant circulating hCoVs.
This is why something like 80% of blood samples taken before SARS-CoV-2 emerged showed T-cell cross-reactivity. From the perspective of our immune system, it was never a "novel" virus. It was novel to scientists and politicians but not to the human immune system.
> There’s nothing silly about a novel virus having greater abilities to overcome barriers when compared to other pathogens that humans have had years decades and centuries to develop defenses again.
Agreed! What you are describing is viral interference: population-level immune responses to one respiratory pathogen causing suppression of others.
We've seen selective suppression of influenza and all of the other four endemic coronaviruses, while SARS-CoV-2 has thrived. This is true throughout the world, including places where stringent horizontal interdictive measures were taken, in places where no such measures were taken, and in all places in between.
> At the end of the day it’s likely gonna be the result of a combination of all these factors.
I think it's very unlikely - almost impossibly so - that interdiction has played a meaningful role in population-level suppression. I don't think it will be a combination; I think it will be explained by the same clearly observed phenomenon which has been responsible for similar outcomes in the past.
Sure, viral interference, yes; but I didn’t go inside a restaurant for 13 months. You’re saying that (and masking, and social distancing in general) didn’t affect influenza transmission?
Although COVID-19 is definitely more virulent than influenza, it doesn't seem reasonable to just jump to the conclusion of saying "it must be the restrictions". That's definitely part of the equation, and maybe it will turn out to be the entire equation, but we don't know.
> During the study period, states allowed restaurants to reopen for on-premises dining in 3,076 (97.9%) U.S. counties. Changes in daily COVID-19 case and death growth rates were not statistically significant 1–20 and 21–40 days after restrictions were lifted. Allowing on-premises dining at restaurants was associated with 0.9 (p = 0.02), 1.2 (p<0.01), and 1.1 (p = 0.04) percentage point increases in the case growth rate 41–60, 61–80, and 81–100 days, respectively, after restrictions were lifted (Table 2) (Figure). Allowing on-premises dining at restaurants was associated with 2.2 and 3.0 percentage point increases in the death growth rate 61–80 and 81–100 days, respectively, after restrictions were lifted (p<0.01 for both). Daily death growth rates before restrictions were lifted were not statistically different from those during the reference period, whereas significant differences in daily case growth rates were observed 41–60 days before restrictions were lifted.
This is one hell of a virus, but if what we are seeing is that it still spreads quite rapidly even with all the restrictions, yet the flu, which is also quite virulent, manages to disappear, then I don't think we really have all the facts as to why it has gone away. Maybe it is just that simple?
I guess what I'm saying is that it's a bit concerning just how fast people here are willing to jump to conclusions.
Yeah, and even in places with no mandatory measures, people would widely take voluntary measures against SARS-CoV-2 that would also be effective against other respiratory viruses.
Whether lockdowns/restrictions are "effective" is somewhat conjecture, but likely had an effect. It's also likely that given the global and nearly total nature of this observed phenomenon that other forces are at play such as the competition and immunity stimulus mentioned in OP.
"Only people who get tested for influenzalike illnesses—typically about 5 percent of individuals who fall ill—are tallied."
If there were a change in the way these tests were administered, for example a blaring medical bias toward another disease, that would present a significant sampling problem. That same statistician also wanted me to mention the simplest and most boring answers are usually closest to the truth.