Let’s talk about antibody testing and herd immunity
Within 1 to 3 weeks after active infection with COVID-19, two types of antibodies start to appear. They are the IgM and IgG antibodies. IgG antibodies generally reflect permanent immunity against a virus. Because we don’t know exactly the accuracy of our current tests, on an individual basis, the value of antibody testing is questioned.
The CDC discourages people from obtaining the antibody test and presuming that they are immune if they test positive for the antibody. Hopefully with further testing will be able to reliably assess the sensitivity and specificity of the antibody test and use it on an individual basis. I’m optimistic that it will be so. This is based on some good news.￼ The good news is that with the SARS virus which presented itself in 2002, patients who did develop IgG antibody had long-term immunity and no evidence of re-infection. And the two viruses acts very similarly to each other. ￼Therefore it is likely that the Covid -19 antibody will provide immunity.
Until we have absolute and better data on the reliability of the antibody testing, the test is being used by centers for disease control for understanding what is described as herd immunity. So while on an individual basis the test may not be very useful, it will be useful in a population.
To understand herd immunity, you have to understand a factor called R(0) which is the transmissibility factor. It asks the question: how many people can one affected patient infect? Data seems to suggest that the number is 2 to 3. In comparison influenza’s R(0) is about 1.5.
To develop herd immunity, the number of people that have to be positive for the antibody is 1-1/R(0).
If R=2 then 1-1/2= 50% So 50% of the population needs to have the antibody for the infection to stabilize.
If R=3 then 1-1/3=67% So 67% of the population needs to have the antibody for herd immunity.
At the present, In New York City which is the most affected city in US, there is 15% positivity for the antibody. Other cities are much lower Therefore we have not received any level of herd immunity.
It Is therefore very important to continue to follow measures of handwashing, wearing a mask and keeping the social distancing. We are all hoping that we will reach herd immunity when the vaccine is out. Or at least when we have treatment options that reduce the patients’ mortality. Until then get out but be safe. And make sure that others are safe from you. This is called social responsibility.