In a study of 100 confirmed CoVID cases in Taiwan, 2,761 close contacts were traced, in which 22 secondary cases were found. That makes for an overall secondary case rate of 0.7%. (Journal of the American Medical Association, May 2020)
When a close contact was within the family of the patient, the attack rates were higher, at 4.6 – 5.3%.
The study did not collect any information about mask wearing, but the cases were in January-March, before social distancing and masks became mandatory in Taiwan (in April).
This is still much lower than the oft-repeated attack rate of “10%” among housemates of an infected person. Ten percent is also about the rate of secondary household cases when a family member has influenza.
A more recent article crunched the numbers of cases that appeared in China starting in January 2020. Their calculations inferred a much higher secondary attack rate, many times higher than the Spanish flu of 1918. By this far in the CoVID pandemic, it would be obvious if this were true.
However that study was only looking at numbers of reported cases, but did not involve actual contact tracing.
Furthermore, the article, written by mathematicians from the Los Alamos National Laboratory, is chock full of qualifying statements to explain the speculative nature of the report, including caveats, inferences and assumptions, as well as relying in part on unpublished data. Their calculations required multiple estimates, some of which the authors admit “contain large uncertainties”.
If close contacts have an overall rate of secondary infection of only 0.7%, where do masks come in?
A study from China reported on the effectiveness of homemade masks composed of polyester cloth plus 3 layers of “kitchen paper”, each paper in turn made of 4 thin layers (paper towels?). The masks were exposed to aerosols laden with a chicken virus. No human subjects were involved.
The masks were reported to block the chicken virus with an efficacy of 95%. It is uncertain how this translates into reducing actual SARS-CoV-2 transmissions rates and subsequent CoVID infections when homemade masks are worn by breathing people, and reused many times.
Although the report makes homemade masks sound pretty good, the study does not describe a real life scenario, which would ideally measure virus penetration through masks worn by patients with confirmed CoVID. There was an attempt to do just such a study, but a technical flaw resulted in retraction of that paper.
It is remarkable that such a simple experiment has not been repeated with adequate virus-detection tests.
Do the math –
- With no information on mask wearing, there was a 0.7% overall secondary case rate in close contacts of persons ill with CoVID, suggesting “no mask” is overall 99.3% effective among close contacts.
- There was a 5% secondary case rate in family members, suggesting “no mask” is 95% effective in family members.
- In a lab study not involving the actual wearing of masks, household masks were found to block a chicken virus in aerosols with an efficacy of 95%. That suggests that masks can be expected (but not demonstrated) to result in lower transmission rates.
Mask-wearing to prevent virus spread should be considered with caution in light of a seemingly opposite idea – the development of herd immunity.
It will take more than a year to develop an effective vaccine, and much longer to test it for safety. Therefore, the population has to largely rely on the tried and true mechanism used since the dawn of life forms. When a population gains experience with a new virus to a point where so many become immune, it protects the “herd”, even those that do not yet have antibodies – thus, herd immunity.
We know from recent random antibody testing that the number of people who met the SARS-CoV2 virus and developed antibodies to it without falling ill is 43 to 85 times higher than previously known.
The antibody studies tell us that a move toward herd immunity is taking place despite mask-wearing and social distancing.
As previously described in this blog, such data makes the CoVID denominator much larger, demonstrating that true death rates are vastly lower than previously calculated.
Many jurisdictions in the US are making masks optional. Hopefully these studies will guide your individual decisions.