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> The likely explanation is that, because the flu is significantly less transmissible than COVID But why is it less transmissible? One obvious explanation is b

> The likely explanation is that, because the flu is significantly less transmissible than COVID But why is it less transmissible? One obvious explanation is because we've lived with it all our lives, and most people already have some level of pre-existing immunity to most flu strains they're exposed to. Both due to natural infections, and vaccines. > In other words, you can probably beat back the flu significantly by having enough of the population mask up, wash/sanitize their hands, etc. If pre-existing immunity is why it's less transmissible, those measures will result in fewer flu infections, and thus lower levels of pre-existing immunity. Which in turn, may make the flu more transmissible, leaving us where we started. Worse, if pre-existing immunity is decreasing the severity of the infections we do get, the net effect may actually be more deaths due to fewer, but more deadly, infections. Flu vaccines are notoriously ineffective, with a typical year being just 50% effective, and bad years being as little as 19%. They're probably the least effective widely used vaccines by a long shot (eg the MMR vaccine is about 97% effective). So we can't just assume we'd be able to make up the difference with increased vaccination. Flu vaccine coverage is already quite high anyway, 51.8% or so for the entire US population. edit: fixed % https://www.cdc.gov/flu/fluvaxview/coverage-1920estimates.htm https://www.cdc.gov/flu/fluvaxview/coverage-1920estimates.ht... https://www.cdc.gov/vaccines/vpd/measles/index.html https://www.cdc.gov/vaccines/vpd/measles/index.html https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htm https://www.cdc.gov/flu/vaccines-work/effectiveness-studies....

khuey 5y ago
Your link says that flu shot coverage is 51.8% for all people over 6 months of age. The 64% number is for children. Flu vaccine effectiveness is not great because they have to "guess" which variants of the flu will be most prevalent months ahead of time due to the manufacturing lead times. A lot of experts are optimistic than the mRNA vaccine technology will lead to more effective flu shots.
- petertodd 5y ago
  Thanks! Fixed. > A lot of experts are optimistic than the mRNA vaccine technology will lead to more effective flu shots. We will see. Influenza is unusual in its ability to evolve new variants that resist existing antibodies. It's also not clear that we even can give people repeated mRNA vaccine shots year after year. People quite often develop antibodies to glycol, which makes up the nanospheres used to get the mRNA into cells. Repeated shots may lead to severe side effects, and/or the nanospheres being destroyed before the mRNA can be delivered, rendering the vaccines ineffective. Incidentally, this is also a problem with the new adenovirus technology used in the Astra Zeneca and Johnson & Johnson vaccines: your immune system develops antibodies to the carrier virus as well as the target proteins. Subsequent shots with the same adenovirus can be destroyed prior to being able to get into cells and express the desired protein; there isn't an infinite supply of suitable adenovirus strains. The Sputnik V vaccine actually uses a different adenovirus for the first and second shots for this reason.
  - selimthegrim 5y ago
    I think this was only a problem with the HIV adenovirus vaccines.
joshuamorton 5y ago
That's not at all an obvious explanation. There obvious explanation is that different diseases have different levels of infectiousness. It just so happens that the symptoms the flu causes make it less virulent than covid-19.
brandon272 5y ago
> But why is it less transmissible? One obvious explanation is because we've lived with it all our lives Haven't we lived with different types of coronaviruses all of our lives as well, though? It seems at least equally as plausible that COVID is more transmissible due to actual physical qualities of the virus with respect to replication rate, particle size, and other factors that cause more virus to be expelled from the body in different ways. It could be that the flu is more transmissible by fomites than through aerosols, or that less flu virus tends to be expelled by the body, or that the flu virus has a more difficult time in infecting our cells compared to SARS-CoV2. Of course, this is pure speculation on my part, but from what I can tell we lack data for anyone to be drawing any firm conclusions on this point.
pmontra 5y ago
Transmissibility (or Basic Reproduction Number - R0) is an intrinsic characteristic of a disease. Measles is at least 3 times more transmissible than flu [1]. We have a very effective vaccine against measles and a mildly effective one against flu, so people get flu and not measles but let them circulate among people without any protection and measles will get them sooner than flu. And it's also much deadlier [2] [1] https://transportgeography.org/contents/applications/transportation-pandemics/basic-reproduction-number-r0-of-major-infectious-diseases/ https://transportgeography.org/contents/applications/transpo... [2] https://academic.oup.com/jid/article/189/Supplement_1/S4/823958 https://academic.oup.com/jid/article/189/Supplement_1/S4/823...
- petertodd 5y ago
  > Transmissibility (or Basic Reproduction Number - R0) is an intrinsic characteristic of a disease. I'm not talking about the theoretical R0, which assumes an immunologically naive population. I'm talking about the real world reproductive ratio, R, including existing immunity. Estimates of real world R for influenza are are even lower than the 2-4 your source quoted. Eg, this paper estimates a median of just 1.3 for a typical year's seasonal influenza: https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471-2334-14-480 https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471... With COVID-19's R0 estimated at 2-3, it could easily be the case that our measures are sufficient to completely wipe out the flu. But not COVID-19: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30840-9/fulltext https://www.thelancet.com/journals/laninf/article/PIIS1473-3... Re: measles, note that pre-vaccine, almost everyone got infected with measles at some point in their life, usually as a child. You actually see this in how effective the early childhood measles vaccination programs were: even though only children were being vaccinated, a small % of the total population, measles rates dropped dramatically(1) almost immediately. The adults had already gotten it and were already immune. So to wipe it out, we just needed to protect those who hadn't been infected: the new, uninfected, children born every year. The measles vaccine is attenuated live measles after all. So by vaccinating, we just get the inevitable infection over with in a controlled way, with the odds rigged _enormously_ in your favor. 1) https://ftp.historyofvaccines.org/index.php/content/graph-us-measles-cases https://ftp.historyofvaccines.org/index.php/content/graph-us...

khuey 5y ago

Your link says that flu shot coverage is 51.8% for all people over 6 months of age. The 64% number is for children. Flu vaccine effectiveness is not great because they have to "guess" which variants of the flu will be most prevalent months ahead of time due to the manufacturing lead times. A lot of experts are optimistic than the mRNA vaccine technology will lead to more effective flu shots.

petertodd 5y ago
Thanks! Fixed. > A lot of experts are optimistic than the mRNA vaccine technology will lead to more effective flu shots. We will see. Influenza is unusual in its ability to evolve new variants that resist existing antibodies. It's also not clear that we even can give people repeated mRNA vaccine shots year after year. People quite often develop antibodies to glycol, which makes up the nanospheres used to get the mRNA into cells. Repeated shots may lead to severe side effects, and/or the nanospheres being destroyed before the mRNA can be delivered, rendering the vaccines ineffective. Incidentally, this is also a problem with the new adenovirus technology used in the Astra Zeneca and Johnson & Johnson vaccines: your immune system develops antibodies to the carrier virus as well as the target proteins. Subsequent shots with the same adenovirus can be destroyed prior to being able to get into cells and express the desired protein; there isn't an infinite supply of suitable adenovirus strains. The Sputnik V vaccine actually uses a different adenovirus for the first and second shots for this reason.
- selimthegrim 5y ago
  I think this was only a problem with the HIV adenovirus vaccines.

joshuamorton 5y ago

That's not at all an obvious explanation. There obvious explanation is that different diseases have different levels of infectiousness. It just so happens that the symptoms the flu causes make it less virulent than covid-19.

brandon272 5y ago

> But why is it less transmissible? One obvious explanation is because we've lived with it all our lives Haven't we lived with different types of coronaviruses all of our lives as well, though? It seems at least equally as plausible that COVID is more transmissible due to actual physical qualities of the virus with respect to replication rate, particle size, and other factors that cause more virus to be expelled from the body in different ways. It could be that the flu is more transmissible by fomites than through aerosols, or that less flu virus tends to be expelled by the body, or that the flu virus has a more difficult time in infecting our cells compared to SARS-CoV2. Of course, this is pure speculation on my part, but from what I can tell we lack data for anyone to be drawing any firm conclusions on this point.

pmontra 5y ago

Transmissibility (or Basic Reproduction Number - R0) is an intrinsic characteristic of a disease. Measles is at least 3 times more transmissible than flu [1]. We have a very effective vaccine against measles and a mildly effective one against flu, so people get flu and not measles but let them circulate among people without any protection and measles will get them sooner than flu. And it's also much deadlier [2] [1] https://transportgeography.org/contents/applications/transportation-pandemics/basic-reproduction-number-r0-of-major-infectious-diseases/ https://transportgeography.org/contents/applications/transpo... [2] https://academic.oup.com/jid/article/189/Supplement_1/S4/823958 https://academic.oup.com/jid/article/189/Supplement_1/S4/823...

petertodd 5y ago
> Transmissibility (or Basic Reproduction Number - R0) is an intrinsic characteristic of a disease. I'm not talking about the theoretical R0, which assumes an immunologically naive population. I'm talking about the real world reproductive ratio, R, including existing immunity. Estimates of real world R for influenza are are even lower than the 2-4 your source quoted. Eg, this paper estimates a median of just 1.3 for a typical year's seasonal influenza: https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471-2334-14-480 https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471... With COVID-19's R0 estimated at 2-3, it could easily be the case that our measures are sufficient to completely wipe out the flu. But not COVID-19: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30840-9/fulltext https://www.thelancet.com/journals/laninf/article/PIIS1473-3... Re: measles, note that pre-vaccine, almost everyone got infected with measles at some point in their life, usually as a child. You actually see this in how effective the early childhood measles vaccination programs were: even though only children were being vaccinated, a small % of the total population, measles rates dropped dramatically(1) almost immediately. The adults had already gotten it and were already immune. So to wipe it out, we just needed to protect those who hadn't been infected: the new, uninfected, children born every year. The measles vaccine is attenuated live measles after all. So by vaccinating, we just get the inevitable infection over with in a controlled way, with the odds rigged _enormously_ in your favor. 1) https://ftp.historyofvaccines.org/index.php/content/graph-us-measles-cases https://ftp.historyofvaccines.org/index.php/content/graph-us...