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Comment by tptacek | Hacker News Reader

Coronaviruses are transmitted in what seems to be the same manner as influenza and generate some of the same symptoms. Why do the worst coronaviruses, like SARS, transmit so slowly, while the flu is practically ubiquitous?

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While SARS does have a longer incubation period, typically 2-7 days compared to the common flu's 1-4 days, the "slow transmission" that you've witnessed is more likely the successful work of national and international public health agencies preventing its spread. Especially by means of contact tracing and quarantine.

Seasonal flu variants have a lethality of around 0.1%. The Spanish flu of 1918 was around 2%. SARS has a fatality rate of around 10%. There is minimal difference in transmission between strains. The only thing preventing disaster is effective public health response.

SARS has a higher fatality rate than 10%. It's more like 25%. The reason why the numbers are so low is because China underreported the number of deaths. Look at Canada for a more reliable number. And that's not including any long term problems the infected people got even after recovering.

My dad was in China during the SARS time. He said they shut down entire hospitals and basically let people die from it. These are things you don't hear about at all.

Having looked through more stats in the last hour, it appears I also dramatically underreported the lethality of the Spanish Flu. Some sources put it at 10-20%. It may have killed 3-6% of the world's population.

>The WHO based its estimates on an analysis of the latest data from Canada, China, Hong Kong, Singapore, and Vietnam. "On the basis of more detailed and complete data, and more reliable methods, WHO now estimates that the case fatality ratio of SARS ranges from 0% to 50%, depending on the age group affected, with an overall estimate of case fatality of 14% to 15%," the agency's announcement said.

>The fatality ratio is less than 1% for people younger than 25, 6% for those aged 25 to 44, 15% for those aged 45 to 64, and more than 50% for people 65 or older, officials said.

>WHO officials observed that calculating the case-fatality ratio for a disease outbreak is difficult while the outbreak is still evolving. The true ratio cannot be determined until the outbreak is over, when the total numbers of deaths and recoveries are known.

http://www.cidrap.umn.edu/news-perspective/2003/05/estimates...

If it is similar to SARS, 19 nCoV could affect developed countries with elderly populations more.

>Of the 298 infected, nearly 230 are being treated in the hospital. Among them, around 60 cases are critically ill. Others who are sick have recovered or been sent home from the hospital. And six people have died.

>It’s possible there are many more people with the virus out there who have very mild symptoms or who are asymptomatic, said Dr. Jeremy Farrar, director of the Wellcome Trust, a research charity focused on global health. If that’s the case, and there are thousands carrying 2019-nCoV while only a few people have died, this outbreak will look milder.

https://www.vox.com/2020/1/21/21075017/sars-wuhan-pneumonia-...

The actual mortality rate will be determined by how many of the critical ill patients recover or die. Long term disability is another thing that needs to be measured.

Just according to superficial stats, it looks like the flu kills orders of magnitude more people than SARS and MERS. Is the public health response to these diseases really so effective as to account for most of this difference? I don't pay close attention but also wouldn't off the top of my head be able to describe anything that, say, Washington State does to effectively respond to CoV incidents to keep it from propagating rapidly through the population the way the flu does.

(I'm asking because I'm curious and seriously have no clue how this stuff works).

Yes, seasonal flu in the US alone kills around 30,000 each year, and may have killed as many as 60,000 during the 17-18 flu season alone [1]. MERS and SARS have each killed less than 1,000 worldwide.

A few things that may interest you:

* The rate of transmission for a disease is estimated by its R0 or basic reproduction number. R0 estimates the number of new cases from the unaffected population that one infection will cause. A disease with R0 <1 will die out by itself. SARS has an estimated R0 of 3-5, while seasonal flus are between 1 and 2 [2]. So by this measure alone SARS is more "infectious" than the flu. Note that R0 is not a rate and is estimated from mathematical models [3].

* Seasonal flu predominantly kills the young, the elderly, and the immunocompromised. Interestingly, some strains, such as the Spanish Flu, have very different mortality curves that kill healthy middle aged adults far more than average [4]. I would expect a dramatically different public health response compared to normal seasonal flu if a future pandemic had a similar mortality curve.

* In the US, the "tip of the spear" for disease response would be the CDC's Division of Global Migration and Quarantine [5].

[1] https://www.cdc.gov/flu/about/burden/index.html [2] https://www.ncbi.nlm.nih.gov/pubmed/19545404 [3] https://web.stanford.edu/~jhj1/teachingdocs/Jones-on-R0.pdf [4] https://en.wikipedia.org/wiki/Spanish_flu#/media/File:W_curv... [5] https://www.cdc.gov/ncezid/dgmq/quarantine-fact-sheet.html

Just according to superficial stats, it looks like the flu kills orders of magnitude more people than SARS and MERS.

This is the "ratio vs. number" distinction: the seasonal flu, despite seeming to have a mortality rate on the order of 1/1000, killed more people just in the US than estimates of people who contracted SARS at all anywhere in the world.

Flu stats: https://www.cdc.gov/flu/about/burden/index.html

SARS stats: https://www.who.int/csr/sars/country/table2004_04_21/en/

When H1N1 was spreading while I was in college, any people who were confirmed cases would have to undergo two weeks of house quarantine. That's a lot of people saved from potential contact. I imagine Washington State's procedures for quarantine aren't too different.

So lots of people get the flu, but few people die from it.

Few people get SARS/MERS, but they're much more likely to die.

There are a lot of reasons for that. And yes, Washington State can effectively respond to CoV incidents - contact tracing, which is what you need to get people into isolation, is very labor intensive, and probably going on right now. State health departments help support hospitals in doing infection control work (to prevent within hospital transmission) and a lot of coordinating.

> Is the public health response to these diseases really so effective as to account for most of this difference?

Yes. The public health response to the common flu is 'You should, like, get vaccinated. Or whatever. Maybe take a day off, if you work a job that gives you sick days.'

Spanish flu was more 20% than 2%

MERS had a 36% fatality rate.

Probably has to do with the conditions it can survive in. For example they may both be able to be spread by coughing, but the coronaviruses may not survive as long at room temperature and therefore have a smaller window to inoculate. Granted, this is purely just my guess as I have no actual idea.

Both of them are enveloped viruses, so differential survival is somewhat unlikely. But there are a lot of things that go into how transmissible a virus is.

I read somewhere that you can get the flu by being exposed to as few as 25 flu virus particles. Maybe the coronaviruses require more actual virus particles?

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