What is a second wave and why do so many predict a second wave of infection? If numbers are going down week on week, why would there be a 2nd wave?
Historically, in most large outbreaks you see basically a growth curve (think of a hill), where you have the run up, increasing until it peaks, then going down (usually as people quarantine themselves). After that, however, you see a second such curve, with a second (or even third or forth) ramp up, peak, and downturn…often the second wave is actually worse than the initial outbreak. People relax, they are relieved that it’s ‘over’, they go back to doing things like they were, and they get another outbreak. You also have the fact that some viruses do worse in the spring and summer, but do really well in the fall and winter. And the fact that while it’s now summer in the northern hemisphere, it’s going into winter in the southern, so the virus basically moves to those latitudes, then comes back when conditions are back to being right for them.
There are a bunch of other factors to this, but that’s the gist. This happens until you either get a large percentage of the population infected who survive and get some level of immunity, or you have an effective and wide spread vaccine. Think about it…the virus is still out there. It’s still as contagious. Why WOULDN’T there be a second wave until and unless you get a large portion of the populace who are immune or resistant.
Because the numbers aren’t going down to zero.
Because the numbers aren’t going down everywhere.
From March through April, there was a great conflagration in the country. Much of the fire was concentrated in the northeast, Washington state, and Chicago and Michigan. But there were medium-sized fires all over. The big fires have died down a little, but the smaller fires have gotten bigger. Like in my wonderful state of Virginia. We’re fixin’ to open back up, because there is no longer any political will left to keep us locked down. Do you honestly think our numbers are going to drop when that happens? Cuz I predict they will shoot up. Which scares the bejesus out of me, I’ll tell you what.
Because as more people think the “numbers are going down” because we have this thing whupped and they don’t have to take precautions anymore, they will get out more, mingle more, have more close contact with NO precautions. And the disease will spread all over again. Because it never went away. In fact, it was the precautions (masks/distancing) that brought the numbers down as long as people were following them. We don’t have this whupped by a long shot. We won’t until there’s a vaccine.
Because of the R value, a measure of how many new infections arise from each existing one. If R > 1 then infections will increase, R < 1 they will diminish. There are several factors which affect R, including how much contact there is between people in the population ©. Another is the susceptibility (S), measured as the fraction of the population who are at risk of the disease. You can reduce C by degrees of lockdown and social distancing, but you can only change S by having a significant % of the population either recover from the disease or through vaccination. You are liable to send R up again, and thus get a second wave, if you relax social restrictions too much before sufficiently reducing S.
Well get yourself some good ole churchin-up, that’ll get the bejesus right back into you and you’ll be fine. 
As you scroll down this page, the very first graph you come to can be sorted state by state. It’s worth a look, especially if you live somewhere where this virus is still kicking ass. Especially especially if you live somewhere and you are unaware that this virus is still kicking ass.
The graph is telling because it tracks deaths. Simply that.
Now that some rudimentary level of testing is becoming available, positive cases look to be skyrocketing. A graph that tracks that is really only going to track testing rates. But deaths have always been countable, and graphable. My state, Illinois is experiencing a small down turn in the number of deaths, but the graph clearly shows that today ain’t much better than we were in the middle of April.
People behaving as if we were past this are going to get a lot of people infected and killed.
Yeppers. It’s still the same virus, and the vast majority of us who still haven’t been exposed have zero immunity to it, so it’s just as deadly to us. Three or four weeks after people loosen up and get out there, I fully expect the body count to start rising again. Or maybe the virus will lose some of its potency in warm weather, in which case (as you suggest) it’ll clobber us in September.
To the greatest degree the experts expect a second wave simply because that’s what most past pandemics have done. Oh they have all sorts of explanations for why that is the dynamic, but bottom line is that.
My understanding is that antibody tests have shown about ~5% of people have been infected with the virus. Which means the other 95% can still get it.
Since it seems to have a high R0, the disease could infect 70-80% of people before it dies out. Who knows if we will have a vaccine before that happens.
One thing I don’t understand about this … what you say makes sense, but, if that is how a pandemic virus behaves, then why did the Spanish Flu eventually stop? Shouldn’t we all still be vulnerable to it? Or if the virus totally disappeared, why and how did that happen?
Didn’t the spanish flu evolve into a less deadly strain?
My understanding is that the war effort and all it entailed (troop movement, people going to work sick, crowded hospitals, malnutrition, etc) allowed the virus to evolve to become more virulent. Generally a virus has to pick between how lethal it is and how infectious, but the war effort allowed the virus to evolve to become both.
After the war ended, the virus couldn’t be both anymore and it evolved to be infectious but less deadly.
If I’m wrong, someone correct me. That was my understanding.
Well, I don’t claim to be even knowledgeable on this, but the short answer is that eventually a population reaches a critical mass of people who have gotten the virus and survived and so aren’t getting it anymore. Remember, there were several waves in the Spanish Flu pandemic (the second wave was actually a lot worse), that lasted several years. Millions died, but many more millions got it and survived and thus got levels of immunity. In addition, people got better at detecting outbreaks, and handling them, and at treating those who had it.
Also, it didn’t totally disappear. It’s an influenza variant that mutated (this is part of the explanation as well, btw…sometimes viruses mutate and become more virulent, but often subsequent generations become less virulent, which I believe was the case with this one), but we still have issues with influenza every year, and it’s a more serious threat than people take it for. No a days, people shrug and just say they have the flu in a similar way to saying you have a cold, but it kills a lot of folks every year.
Anyway, that’s my non-scientific explanation. I’m sure more knowledgeable 'dopers will be along for much more ind-epth and technical explanations.
They keep talking about a second wave in California, we have not had a first wave yet. I don’t know what technically should be considered a wave but I would think a bare minimum of 10% infections. 20% to 30% spread over a several months should be manageable especially if pains were taken to guard the vulnerable. Waves after that should be progressively not as aggressive as most of the main carriers would be infected first.
A wave is something that has the shape of a wave. It builds, crests and falls.
A really good look at how epidemics work, and how you can have multiple waves and the interaction with control measures are these two videos from 3blue1brown. The first provides a lot of the basis for the second, which includes demonstration of waves.
https://www.youtube.com/watch?v=gxAaO2rsdIsAs you are warned in the video, these are simple simulations, so the actual numbers are just pulled out of the air, but the principles are well illustrated.
As noted above, viruses can mutate. Influenza does this a lot. A strain of a virus can mutate into a new virus, which is why we have continuous problems every year. But just what you call a “new” virus is open to discussion. From the point of view of humans, a new virus is one that we have not got any immunity to. That is arguably an artificial distinction. It ultimately defines a virus based upon its coat’s signature. (Which is what the HxNx definition is all about.) But a virus can mutate in other ways. In particular, its virulence can change with mutations, even if its coat does not.
Such mutations compete with one another. Because they have the same coat, a human only catches that form of virus once, so the most successful version is the one that out competes the other mutations for the largest number of infected humans. Once one has infected a human, the other mutations are out of luck. Highly virulent mutations are not the best adapted for such success. They tend to strike down the infected subject early, and in the worst case kill them before the subject has a chance to infect others. But the less virulent mutation cheerfully gets passed from human to human subject. Even thought people get sick, they are not so sick that they cannot pass it on. So once a less virulent mutation occurs it will out compete its virulent predecessor. Eventually it may extinguish that predecessor in the wild.
So, what happened to the Spanish flu? Arguably it never went away. It is probably still infecting us now, but it mutated into a less nasty form that successfully out competed the deadly form well enough that the deadly form has been extinguished. As far as our immune system is concerned, since both the deadly and mutated form have the same viral coat, they are still the same virus, and even now, anyone, once infected by the more benign mutated form, would be protected from infection from the deadly form - if it was somehow revived.
The Spanish Flu was caused by an H1N1 flu virus. There are still H1N1 viruses out there that cause flu epidemics, the most recent one in 2009. Although from what the CDC says, that one was significantly different than previous H1N1 epidemics.
As a general rule, diseases tend to mutate towards less lethality. The reason is that they are essentially parasites and killing one’s host is not a successful move for parasites. So if a strain appears that kills fewer of its hosts, it’ll outcompete those that kill more of them.
Is it known that once you get it and survive that you are immune from getting it again? I know that’s typical for viruses, and basically how vaccines work, but I think I’ve heard of a few cases where people got it twice (I could be wrong).
If this mutates again and again like the cold virus, but is deadly, we got some real problems.
That doesn’t actually logically follow. Yes, a long running virus is probably one that is less lethal but that doesn’t mean viruses tend to mutate that way. The ones that do mutate that way are just more likely to survive.
I don’t understand this. A virus either kills its host and then dies off (those individual viri), or the host kills it first. How does either give a virus an evolutionary benefit?