In a recent Senate hearing, Senator Rand Paul of Kentucky in a question to Dr. Anthony Fauci suggested that the reason for the current relatively low rate of infection in New York City is herd immunity.
Dr. Fauci objected saying that Paul was not listening to the director of the CDC who had mentioned that in New York, the positivity rate is about 22% and further suggested that if the Senator believed that a 22% positivity rate results is herd immunity, he believed that he would be alone in that.
Paul is a Senator with no specific expertise in anything while Dr. Fauci is a world renowned epidemiologist. Nevertheless, I believe that Senator Paul is basically right.
First, the Senator is not completely alone in believing that a 22% of a population having been previously infected provides significant protection in a heterogeneous model.
In a homogeneous model that assumes that everyone is alike in that regard, the percentage must be relatively high. However, it can be lower in a heterogeneous model that results from infection due to a disease since some members of a population will be more likely to get the virus than others whether due to circumstance or behavior.
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 as likely on average 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. For this reason, 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.
How herd immunity happens, and will it protect us from coronavirus?
Further to the arguments presented in the article, I would mention that herd immunity is not a policy to be pursued. Instead it is a natural epidemiological phenomenon. It is simply the case that when relatively large numbers of people are infected by a disease and are therefore naturally immune from said disease, it reduces further infection.
By definition, herd immunity represents a reduction in the risk of infection with a specific communicable disease (such as measles or influenza) that occurs when a significant proportion of the population has become immune to infection so that susceptible individuals are much less likely to come in contact with infected individuals.
Finally, we can look at the New York experience itself to look for evidence. On March 16, New York state shut down along with the rest of the nation after the federal government issues new guidelines which were said at the time to be for the purpose of flattening the curve.
At that time, New York had 235 reported cases. A week later, with all nonessential workers sent home, the number of news cases rose to 2,862 per day. After two weeks while shut down, the number of news cases rose to 6,875 per day. After three weeks while shut down, the number on new cases rose to 9,432 per day. On April 10, nearly four weeks after the shutdown, the 7 day average reached 10,052 new cases per day of the coronavirus after which the number of reported new cases started declining. It eventually dropped to the 800-1000 range where it has remained for the past four months.
The question is whether the eventual decline was a result of the policy that led to the shutdown or was it because a significant number of subway riders (many of whom were probably asymptomatic) became infected and the virus began to find it increasingly difficult to find uninfected people to infect. (Kind of like the law of diminishing returns for the virus.)
I tend to the latter view. I believe that social distancing is effective and I support the wearing of masks when social distancing is not possible and particularly inside public buildings such as grocery stores where ventilation may not be perfect. But I think the shutdown was basically ineffective and I also think that continuing the shut down (which is still occurring in some states) is not justified by the science.
www.washingtonexaminer.com
www.worldometers.info
www.snopes.com
www.telegraph.co.uk
www.medrxiv.org
www.medpagetoday.com
