By now it is probably apparent that SARS-CoV-2 is making a comeback in the Northern Hemisphere. Why now? There is no good answer to that, but in my opinion a mix of three aspects will be partly involved. The first is a bit of complacency. People who have avoided getting infected for a few months tend think they have dodged the bullet. They would have, but soldiers know that you cannot keep dodging bullets forever; either you do something about the source or get out of there. In the case of the virus, sooner or later someone with it will meet you. You can delay the inevitable by restricting your social life, but most people do not want to do that forever.
The second may be temperature. Our Health Department has recommended that places where people congregate and have heating systems should raise the temperature to 18 degrees C from the 16 currently advocated. Apparently even that small change restricts the lifetime of the virus adhering to objects, and viruses exhaled have to settle somewhere. This won’t help from direct contact, but it may prevent some infections arising from touching some inert object. That can be overcome by good hygiene, but that can be a little difficult in some social environments. My answer to that is to have hands covered with a gel that has long-term antiviral activity. (Alcohol evaporates, and then has no effect.)
The third is the all-pervasive web. It seems to be unfortunate that the web is a great place for poorly analysed information. Thus you will see claims that the disease is very mild. For some it is, but you cannot cherry-pick and use that for a generalization. If you say, “Some, particularly the very young, often only show mild symptoms,” that is true, but it identifies the limits of the statement. For some others the disease is anything but mild.
A more perfidious approach is the concept of “herd immunity”. The idea is that when a certain fraction of the population have been infected, the virus runs out of new people to infect, and once the infection rate falls below 1 it means the virus cannot replace itself and eventually it simply dies out. Where that value is depends on something called Ro, the number of people on average that the virus spreads itself to. This has to be guessed, but you see numbers tossed around like herd immunity comes when 60% of the people are infected. We then have to know how many have been infected, and lo and behold, you find on the web that a couple of months ago estimates said we were nearly there in many countries. The numbers of infections were guessed, and given the current situation, were obviously wrong. It is unfortunate that many people are insufficiently sceptical about web statements, especially those where there is a hidden agenda.
So, what is the truth about herd immunity? An article in Nature 587, 26-28 (2020) makes a somewhat depressing point: no other virus has ever been eliminated through herd immunity, and further, to get up to the minimum required infection rate in the US, say, will, according to the Nature paper, mean something like one to two million deaths. Is that a policy? Worse, herd immunity depends on the immunity of those infected to remain immune when the next round of viruses turn up, but corona viruses, such as those found in the common cold, do not give immunity lasting over a year. To quote the Nature paper, “Attempting to reach herd immunity via targeted infections is simply ludicrous.”
The usual way to gain herd immunity is with a vaccine. If sufficient people get the vaccine, and if the vaccine works, there are too few left to maintain the virus, although this assumes the virus cannot be carried by symptom-free vaccinated people. The big news recently is that Pfizer has a vaccine they claim is 90% effective in a clinical trial involving 43,538 participants, half of which were given a placebo. (Lucky them! They are the ones who have to get the infection to prove the vaccine works.) Moderna has a different vaccine that makes similar claims. Unfortunately, we still do not know whether long-term immunity is conveyed, and indeed the clinical trial still has to run for longer to ensure its overall effectiveness. If you know you have a 50% chance of getting the placebo, you may still carefully avoid the virus. Still, the sight of vaccines coming through at least parts of stage 3 trials successfully is encouraging.