Why other viruses causing a pandemic?

[Dandy008]

It’s late and my brain is probably just over thinking….

Why aren’t other viruses causing a pandemic?

Obviously we have the flu vaccine, so it’s that what stops the flu from just spreading like Covid?

If you’re ill will flu why can’t you legally go out and to work (I mean morally it’s not great) but if you have Covid you must stay indoors?

Did all viruses start out as a pandemic? What stops then from just speeding?

Well I'm sure lots of cold viruses are pandemics but if they only cause the common cold no one would know or care. Flu pandemics are a thing, and there are scares about those every few years.
There was an Ebola pandemic in West Africa a few years ago, which looked at one point as if might get seriously out of control.

To get pandemic status you need something that is serious enough so you notice, highly transmissable and which most people or at least a lot of people are susceptible to. A novel virus is just the thing but lots of novel viruses just arent that good at spreading and die out (thankfully).

Oh and to answer your question about flu, there are different types, most of us have had several and have at least partial immunity to them. And for the elderly and vulnerable there are vaccines. Even then they kill a fair few in bad years but I guess we are used to that.

Flu isn’t a pandemic because we’ve all had it before numerous times so we have some immunity already. If you took flu to a country where it never existed before, where people had no immunity, then yes it would be a terrible pandemic. Which is exactly what happened when European people colonised the Americas and brought viruses with them. Covid is a pandemic because it’s novel, because nobody had any immunity so everyone would get very sick when catching it for the first time.

Also there have been flu pandemics in the past, most notably the Spanish Flu in 1918. It was also a novel virus, a brand new type that nobody had immunity to, and it was disastrous. But viruses tend to evolve and become less virulent as time passes, lots of our fairly mild modern flu viruses are descended from the Spanish Flu virus.

Novel viruses probably did cause a lot of deaths but the global spread was probably far slower. The flu pandemic of 1918 was spread by the mass movement of people at the end of WW1. When a smallpox outbreak happened in the 50s, people were under obligation to report and go into isolation hospitals.

Surely you realise we've had multiple pandemics in the past?
We've either developed a vaccination or natural immunity, or our living conditions have changed to the point that the disease is less likely to thrive.
Bubonic Plague wiped out 50% of Europe, Spanish Flu, 50 million, we're still fighting the AIDS pandemic, although treatment and awareness has really improved

flu WAS a pandemic when it started out

The book Spillover (David Quammen) gives a really good explanation of viruses and how zoonotic viruses pose a greater threat than ones that humans have lived with for a long time.

A somewhat over-simplified explanation:

A pandemic is an epidemic that affects multiple countries.
A epidemic is when there is an outbreak of a disease above the baseline/endemic level.
When a disease is endemic, there is a constant number of cases, either all the time or in a repetitive pattern eg a seasonal pattern.

Most viruses are endemic. The reason for that is immunity. You can only infect people who are susceptible. If lots of people are immune, infectious people have a lower chance of being in contact with someone who is susceptible. If enough people are immune, you reach the herd immunity threshold because most infectious people won't come into contact with with a susceptible person.

When a disease is endemic, instead of having big waves of infection like we have had, you get to a point of equilibrium where the number of cases is constant because every time a person in a population/sub-population becomes infected and gains immunity, another person in that population becomes susceptible because their immunity has waned or a baby is born without immunity or someone migrates to that area who has not been exposed to the disease etc

You can also get epidemic cycles eg measles. Infection with measles usually confers lifetime immunity. Before vaccination, measles is so infectious that nearly everyone who was susceptible would catch it during an outbreak and the herd immunity threshold would be reached in the local population, it would die out locally. Then, once enough babies had been born that the number of immune people in the population was below the HIT, someone who was infectious would visit the area and there would be another epidemic.

When swine flu was about in 2009 people had to isolate, have medicine dropped off t their front doors and pregnant women were recommended to stay away from crowded places. I was pregnant at the time so remember being very worried about it. If I remember rightly it affected young people worse and several pregnant women had died and couldn't have the cure.

There could still be another flu pandemic if a particularly virulent/fatal strain develops. One of the initial difficulties with the COVID pandemic was that any pandemic planning that had been done was based on the assumption that the next pandemic would be flu-based.

If you have the flu, you wouldn't be physically able to work at the most contagious part...you'd be lucky to be able to walk to the car letalone work.

@housemdwaswrong

If you have the flu, you wouldn't be physically able to work at the most contagious part...you'd be lucky to be able to walk to the car letalone work.

I was just about to say this. I had flu as a teenager and I couldn't even get out if bed for the toilet let alone go to work!

@housemdwaswrong

If you have the flu, you wouldn't be physically able to work at the most contagious part...you'd be lucky to be able to walk to the car letalone work.

Many people were tested positive with swine flu in 2009 while suffering mild to moderate symptoms or no symptoms at all - perfectly fit to do the school run, go to the cinema or do undemanding office work. I'd have thought that if the last year had taught us anything it would be that the exact same virus can affect some people very seriously indeed but some people far more mildly.

Anyway, the developed world has spent the past thirty years planning for an inevitable flu pandemic, which would be caused by a novel, probably zoonotic, variant which might rip through a population with no pre-existing immunity.

Why aren’t other viruses causing a pandemic?
To answer this question simply @Dandy008 it's because a virus needs to have certain characteristics which allow it to cause a pandemic.

It needs to be able to transmit from one host to another (so if a virus is airborne it'll transmit 'better' than one that transmits through body fluids)

It needs to have a period of time where the person who is infected with it is not ill. Because where a host person is ambulant for longer they have more time to spread it to other people. Also if the virus doesn't make everyone ill then there's lots of people able to walk around passing it on (unwittingly)

If a virus mutates frequently enough there's less immunity built up and can infect people several time and it's harder to make a vaccine for it

Covid has a lot of these things so is idea for a worldwide pandemic. Something like Ebola won't be a worldwide pandemic because someone who has it become ill quite quickly and can't manage to walk around and spread it.

Flu occasionally causes pandemics because of antigenic shift. This is where a major new strain emerges. IIRC it’s usually when a strain has jumped across the species barrier for the first time (as with Swine flu which came from pigs). This means that no one has any prior immunity to it, so there is the potential for a large proportion of the population in many countries to become ill, all within a short space of time. This jump from another species is what’s thought to have happened with COVID-19.

In a normal year, we get antigenic drift with flu - which is just the emergence of slightly altered strains. Because of this and waning immunity, people who are vulnerable are offered a vaccine, but most of us have had flu before (probably several times, possibly without realising because you don’t always get severe symptoms) and will have some immunity from having been infected with similar-ish strains in the past. So these slightly altered strains don’t cause a pandemic.

@Finzi

Why didn’t Swine Flu cause a Global Pandemic on the same scale as Covid?

Is that because of the pre existing immunity to flu?

I don’t know. Probably something to do with factors like the R value, the length of time people are infectious for before they feel ill and take themselves out of circulation, the proportion of cases which are asymptomatic etc. These things will all make a difference to how quickly and how far a disease spreads. I think Swine flu was classed as a pandemic even so. SARS and MERS were coronaviruses that potentially could have caused major pandemics but they just fizzled out. I think in those cases the epidemiologists think it was because they were less contagious. Chris Whitty did a Gresham lecture back in 2018 ish about pandemics which talks about this sort of stuff. I think it’s still available to view on the Gresham College website.

[quote Dandy008]@Finzi

Why didn’t Swine Flu cause a Global Pandemic on the same scale as Covid?

Is that because of the pre existing immunity to flu?[/quote]
Finzi has described a lot of the reasons - pre existing immunity played a huge role, as did the fact that we already had an effective vaccine that could be tweaked.

There's a short series on Netflix called Coronavirus explained which would answer most of your questions. It's in simple terms so totally understandable for people who don't have any previous medical or virology background. I watched it with my 13 year old and she was fascinated.