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1918 Influenza Pandemic

Because COVID-19 produces a “flu-like illness,” there is a lot of confusion as people relate this disease to the flu. A Facebook friend recently asked for clarification. This is my response. Note: in the two days since I drafted this, some information changed.

It’s important to separate flu from COVID-19. While both produce a flu-like illness, they are from different virus families.

Let’s start with COVID-19 or the virus that causes it: SARS-CoV-2.

It is from the family Coronaviridae. It’s also an RNA virus but that’s where the similarity ends. But it’s new, hence the fear. While we can study the viral genome, we don’t yet know fully how it’ll behave in humans. Genetically, it is most similar to the SARS virus, which terrified many of us in 2003 when it spread from China to Canada. SARS had a fatality rate of 14–15% and an R0 between 2–5. That is a very high death rate for a respiratory virus that can quite easily be transmitted, but due to some stringent public health actions, the virus was contained. This may also be because it was so deadly. It killed its hosts before they could spread it.

For SARS-CoV-2, the fatality rate has been about 2–3%. Higher than for flu but lower than SARS. We are not 100% certain of the R0, but it appears to be around 1.4–2, again, higher than flu, but lower than SARS.

SARS-CoV-2 is also mostly spread via droplets in sneezes and coughs. It appears to also live in fomites, like flu. So far, airborne transmission doesn’t appear to be likely.

So far, COVID-19 seems to be worst for older people, while kids seem to be less likely to get very sick. Note that the flu virus varies in how it affects people. Some years, like this one, influenza is hardest on the young adult population. Other years, it mostly affects the elderly or very young.

For SARS-CoV-2, most of the fear is based on the unknown. Current models show it spreading rapidly. Rapid spread will quickly overwhelm existing health care systems which will lead to a higher mortality rate. COVID-19 is spreading faster than SARS. Could that be because the illness is more mild? What’s clear is this:

1) We need more extensive testing capacity,

2) We need funding for disease surveillance,

3) we need competent public health management at every level, and

4) experts need to be free to share their information.

Now, back to Influenza.

People tend to call any respiratory illness flu, but actual flu is caused by the influenza virus, which is from the family Orthomyxoviridae. There are four types. Type D only infects cattle. Type C is very mild and we rarely hear about it. Type B infects seals and humans and have never caused pandemics. It’s also usually mild, except this year where we’ve seen mostly Flu B and some pretty significant illness. And then there’s Type A, broken into multiple subtypes based on it’s surface proteins H and N. For seasonal flu, the case fatality rate can vary between 0.1% to 0.3%.

The influenza virus is a segmented RNA virus. That matters because the segments can re-assort. Because Type B only infects seals and humans, and it’s pretty rare for seals to cough on humans and vice versa, the B viruses don’t have an opportunity to mix and re-assort. So Type B viruses don’t change much. They can mutate and we end up with different “lineages” but they’re fairly consistent year to year.

Influenza A strains can infect humans and various animals, including birds. It does not re-assort with Influenza B, but does re-assort with other A strains, and that’s why it often changes from year to year. Sometimes, that change in surface proteins leads to a new form of Type A virus. These novel viruses have lead to three pandemics in the 20th Century: 1918 (H1N1 of avian origin), 1957 (H2N2), and 1968 (H3N2). Most of us remember the 2009 pandemic (also H1N1 but slightly different from the 1918 version). The case-fatality rate varied dramatically for each pandemic. Some of that was due to advances in medical care, some due to the lethality of the strain.

Bird flu (H5N1) scared many of us especially around 2005–2006, but it did not cause a pandemic. That’s because it can’t be transmitted between humans. Humans get it from birds, but they can’t give it to each other. This is known as a dead end host. Unfortunately, the case fatality rate (60%) for H5N1 is very high. That terrified us but it never mutated to spread between humans.

Until recently, the World Health Organization (WHO) had three criteria to declare a pandemic:

  1. The emergence of a disease or pathogen new to a population
  2. The pathogen infects humans, causing widespread, serious illness
  3. The pathogen spreads easily and sustainably among humans

The WHO has recently changed its criteria and has not yet declared COVID-19 a pandemic (this is likely to happen soon).

Then there is the level of contagion, or R0. Seasonal flu has an R0 of about 1.2, or five people can spread it to six people. Compare mumps: the R0 is 6–7, for measles, it’s 18–19.

The incubation period is the time between when a person is infected and when they get sick. For influenza, it is one to four days. A person can give it to another before they show symptoms. Influenza is mainly transmitted by droplets — i.e. you cough or sneeze and the virus is in tiny droplets that land in a person’s mouth or nose. Fomite transmission — where the virus lives on surfaces and you pick it up on your hands — can also occur. Compare measles with its high R0 value — that virus is mainly airborne.

To prevent the flu, you can get a flu vaccine yearly. While not perfect, it usually prevents against serious disease.

Here are some trusted sites for information:

https://www.idsociety.org/public-health/Novel-Coronavirus/
https://systems.jhu.edu/research/public-health/ncov/
http://www.cidrap.umn.edu/

Dr. Rohr-Allegrini is an epidemiologist and tropical disease scientist currently working to prevent diseases through immunizations.

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