Lately I have been giving some additional thought to the possibility that we may be nearing the point of herd immunity in certain areas of the country. It appears from the charts that new reported cases for the states of Arizona, Texas and Florida are finally headed downward. If they are truly headed downward as it appears they are, my guess is that it is mainly due to herd immunity. The charts for each of those states may be accessed from links on the following page:
https://www.worldometers.info/coronavirus/country/us/
Here is the argument:
About a month ago, on June 25, Dr. Robert Redfield, director of CDC, indicated that antibody testing has indicated that for every case that was reported, there were actually 10 other infections that had not been reported. At that time, about 2.4 million cases had been reported which means, according to the analysis, that 24 million Americans had contracted the disease.
https://www.forbes.com/sites/mattpe...-than-are-diagnosed-cdc-reports/#3196f0707293
Recently, a larger study has been released that essentially confirms the earlier study on which Dr. Redfield based his statement. This study, as well as the earlier study, is somewhat dated, however, in that it is based on residual clinical specimens collected between March 23 to May 12. A link to the study is given below:
https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2768834
Obviously, the study is valid only for the period in which the specimens were collected. Since then, the number of tests for the coronavirus has increased significantly. Back in May, the US was performing about 300,000 tests per day whereas now, we are performing about 750,000 tests per day. But the number of reported cases has almost tripled from about 26,000 to almost 72,000 per day. It is clear that testing cannot account for most of the increase in the number of new cases particularly since we are doing more repeat testing than we were back then.
Thus it might seem plausible to assume that the increase in testing has probably not significantly affected the ratio of reported cases to actual cases. Perhaps therefore, we can assume as many as 40 million Americans may have now been infected with the coronavirus.
This works out to about 12% of the population with prior infection and the question arises: what is the proportion of the population that must have had prior infection in order for the country to experience herd immunity?
Models assuming homogeneous distribution of immunity have typically put the herd immunity level for the coronavirus at around 60 percent. Homogeneous distribution of immunity is considered appropriate in the case of a vaccine since vaccines tend to spread the immunity evenly throughout society.
There is, of course, considerable distance between 12% and 60%. However, the percentage may be reduced if a heterogeneous basis is assumed since a heterogeneous basis considers how people mix or connect within a population.
The percentage in a heterogeneous model may be lower since some members of a population will be less likely to be infected with virus whether due to circumstance or behavior. People who work at home are about as likely to seek vaccination as people who travel to work via a subway system.
But in the case of people gaining the immunity via disease, people who travel to work on a subway system are much more likely to be infected than people working from home or traveling by private car to work. The threshold for herd immunity is lower because behavior affects how the disease may spread in the population. Some modelers have used 43% in heterogeneous models. Others such as Gabriela Gomes of The University of Strathclyde have used 20% as the threshold for herd immunity.
https://thehill.com/changing-americ...-herd-immunity-happens-and-will-it-protect-us
But even 20% is some distance from 12%.
However, two things must be noted. The actual percentage for effective herd immunity is not known. Some modelers may have a better understanding of it than others, but they are all guessing. Second, the effects of herd immunity are not suddenly felt within a given population when a certain percentage is arrived at. Its effects are gradually acquired. And regardless, it appears that there are example in which certain populations have gained herd immunity with relatively low percentages. Sweden appears to be an example.
Sweden has been heavily criticized for not doing enough to prevent the spread of the virus. However, if one looks at the graph for new cases in Sweden and checks the 7 day running average checkbox, one will notice that the 7 day moving average for new cases in Sweden have fallen from 1,123 on June 29 to 249 on July 22. Furthermore, number of new cases appears to be headed further downward. Also, the number of deaths have fallen from 95 per day in mid-April to 3 per day. The chart is found at the following address:
https://www.worldometers.info/coronavirus/country/sweden/
What did the Swedes do to achieve this result?
Apparently nothing except to allow the disease to take its course. They encouraged social distancing and the wearing of facial coverings, but they mandated neither. They never closed the bars and restaurants. They never shut down other businesses. Presumably, some people stayed at home due to fear of the virus, but the government mandated nothing.
The Swedes did, of course, have an advantage. They were off the beaten path. Unlike other European cites, Stockholm is not a major international travel hub. As a result there was no significant infusion of outside travelers. Thus they never had much more than a thousand reported new cases per day. Deaths maxed out at less than a hundred and continued to dwindle with time.
So what is the percentage that achieved herd immunity in Sweden?
Well, Sweden has a population of about 10.1 million people and they have had a total of 78,500 reported cases as of yesterday which yields 0.77%. If it is assumed that Sweden had an actual 10 cases for each one reported, that would yield 7.7%
One may guess that the percentage of bar patrons and regular restaurant goers who have been infected with the virus is much higher than 7.7%, however.
