(mods, posting here as I am looking for a factual answer. Please move to QZ if you deem it more appropriate.)
Since the.beginning of the pandemic, my family and I have been pretty much in isolation. The only social extravaganza that we entertained has been continuing to send our 2yo (now 3yo) to a daily playgroup, so that he would learn how to relate to other human beings in the flesh other than his parents.
During this period of time, the lovely critter has brought home at least 4 or 5 colds, which is not very different from what happened during the previous year. Apparently, though, and fortunately, he did not bring home any Covid.
Being the ignorant that I am, I implicitly assumed that this could be explained away by the common cold having a much higher transmission rate, but apparently that is not the case:
So I am left wondering, why is this the case? What are the epidemiologic factors that I am not considering and that would explain why the same sample of a population can be repeatedly be exposed to a (family of) virus(es) and not to another one, even though they are both circulating at the same time and have very similar R0?
Notes: I understand that we are a small sample, but I know many other people who got several colds this winter and no Covid. Also the larger sample may be biased, so I would be happy to learn of other dopers have similar or.contrasting anecdotal or scientific evidence.
The actual reproduction number R is the one that counts. R = R0 times the susceptibility S, which is the proportion of the population who are not immune. Many people will have already been exposed to many different colds so S is likely to be somewhat lower.
Secondly, R0 can be reduced by taking precautions such as isolating when infected, that’s why lockdowns work. I doubt parents are as scrupulous in isolating their children just for common colds.
But then, given the same R0, there should be more people around with Covid than with Cold, right?
This could have played a role. However, children have been described to be by and large asymptomatic w.r.t. Covid, and in general parents have been instructed to keep their children at home at the first sign of illness. In fact, the caregivers told us that they never had as few runny noses as this year.
A big factor is probably that the common cold is much more transmissible via surface contacts than COVID is. Early in the pandemic everyone assumed surface contact would be a major vector of infection, but that doesn’t seem to be the case. So if your kid is wearing a mask, and so are the others around them, the risk of COVID via airborne infection is low, but if they’re sharing tables and chairs and doorknobs and other such things, they still have a good chance of picking up a cold.
By the way you raise a very interesting point. In more than one occasion I am quite sure that the little one got sick after a visit to a (deserted) playground rather than at the playgroup. Do you have a ref comparing the contact transmission rates of the two viruses?
Kids in general seem to do a fairly poor job at transmitting SARS-CoV2. It seems be mostly airborne transmission with a minority of all those infected doing the majority of the transmitting, and that minority is not heavily weighted to kids as efficient spreaders.
Many other common kid viruses are surface transmission much more and if there are children in the environment, especially young children, just accept the fact that, no matter how much you sanitize, every surface of that environment is covered with a microscopic layer of snot and poop.
Mask or not hand to nose to tush to mouth to surface to …
No direct comparison, but there’s this article from a few years ago:
Because common colds are caused by a plethora of viruses, research on surface infectious rates are harder to nail down. In general, most are no longer dangerous after 24 hours, and their ability to infect dissipates faster on porous materials like facial tissues.
Quantitative microbial risk assessment (QMRA) studies have been conducted to understand and characterize the relative risk of SARS-CoV-2 fomite transmission and evaluate the need for and effectiveness of prevention measures to reduce risk. Findings of these studies suggest that the risk of SARS-CoV-2 infection via the fomite transmission route is low, and generally less than 1 in 10,000, which means that each contact with a contaminated surface has less than a 1 in 10,000 chance of causing an infection 7,8,9.
But it also says:
Data from surface survival studies indicate that a 99% reduction in infectious SARS-CoV-2 and other coronaviruses can be expected under typical indoor environmental conditions within 3 days (72 hours) on common non-porous surfaces like stainless steel, plastic, and glass 10,11,12,13,15. However, experimental conditions on both porous and non-porous surfaces do not necessarily reflect real-world conditions, such as initial virus amount (e.g., viral load in respiratory droplets) and factors that can remove or degrade the virus, such as ventilation and changing environmental conditions 8,9. They also do not account for inefficiencies in transfer of the virus between surfaces to hands and from hands to mouth, nose, and eyes 8,9. In fact, laboratory studies try to optimize the recovery of viruses from surfaces (e.g., purposefully swabbing the surface multiple times or soaking the contaminated surface in viral transport medium before swabbing). When accounting for both surface survival data and real-world transmission factors, the risk of fomite transmission after a person with COVID-19 has been in an indoor space is minor after 3 days (72 hours), regardless of when it was last cleaned 8,9,10,11,12,13,15.
Thank you so much @Horatius and @DSeid , it all makes sense. To summarize, the two viruses have comparable R0s but this factor is unevenly distributed over multiple transmission vectors, Covid being more efficiently delivered via aerosol. That explains why in environments with many kids who are in the habit of sliming everything with snot, cold would be spread very easily. Conversely, kids being by and large asymptomatic, when it comes to Covid, they do not pass it around very easily. İgnorance fought!
