Recently the results of SARS-CoV-2 (new coronavirus which causes COVID-19) antibody testing on people in different parts of the world were reported, and much attention was given to the possibility that the number of those carrying the antibody for SARS-CoV-2 might be more than several tens of times more than the number of patients who are actually infected by SARS-CoV-2 (PCR positive). In an earlier post, I reported on a study that found that 17.8% of the passengers who tested PCR positive on the cruise ship docked in Yokohama harbor had asymptomatic infections. Many readers may be puzzled by these findings.

Here I would like to explain the difference between antibody testing and PCR testing and explain the contradiction in the numbers.

First of all, PCR testing looks for the current presence of the virus (antigens) in the body. It is the most direct way of finding whether infection has occurred.

On the other hand, antibody testing looks for whether SARS-CoV-2 antibodies have been made by a person's immune system, or more correctly, whether the antibody titer or level has risen. It is a test that shows whether the person was exposed to SARS-CoV-2 in the past, not the current state. A person with a rising antibody level indicates that SARS-CoV-2 (antigen) no longer exists in the body.

It takes about two weeks for the antibody level to rise after SARS-CoV-2 has entered the body.

For example, according to the results of antibody testing in Santa Clara County, California, 2-4% of those tested showed an increase in antibodies levels. This indicates that after the COVID-19 infection began in the county, 2-4% of the population were infected. Given that the population of Santa Clara County is 1.92 million, this means that between 40,000 and 80,000 people had been infected in the past. This is equivalent to between 20 and 40 times the number of about 2000 infected people as of the end of April 2020. In this case, COVID-19 infection includes both those with symptoms and with no symptoms (asymptomatic infection). As the survey in Santa Clara County was conducted of asymptomatic volunteers, there is a tendency to think that they were all cases of asymptomatic infection.

Those who are infected with SARS-CoV-2 show a wide range of symptoms ranging from asymptomatic to severe pneumonia or conditions resulting in death. Those with mild symptoms have only a slightly sore throat, a low fever, or a loss of the sense of smell or taste. It is possible that a significant number were included in those who volunteered to take the above-mentioned antibody test and were asymptomatic. They were not typical patients with the classic symptoms of a fever lasting four days or more, difficulty breathing, or of course, an asymptomatic infection that displays no symptoms at all. Incidentally, in a survey of those who were on the cruise ship in Yokohama harbor, even those passengers who showed slight symptoms were categorized as symptomatic.

It takes at least two weeks for the antibody levels to increase, so if the COVID-19 infection had not yet shown an explosive surge, people with mild symptoms such as a slightly sore throat, slight fever, and loss of the sense of taste would not have sought medical care and would not have been counted as infected, or they would probably have been diagnosed at medical care facilities as having a common cold, and of course, not received a PCR examination.

In this way, the fact that at present 17.8% of those infected (PCR positive) have asymptomatic infection and that so far those who have contracted COVID-19 up to now (those with asymptomatic infection + patients with slight symptoms) account for 2% are not necessarily contradictory if many of them did not seek medical care due to mild symptoms. Nevertheless, there are many people who will not be convinced by such an explanation alone. In fact, the antibody test also has a problem that is explained below.

According to a recent news report, 14% of New York residents are antibody carriers, making it the state with the highest percentage. This means that there are many times more people who are asymptomatic or who show slight symptoms than those who are symptomatic. When the percentage of those having antibodies reaches a certain percent (60%), the region is said to have attained herd immunity, the point at which that the virus will not spread further, and if this information is correct, it is welcome news because it means that end of the COVID-19 pandemic is in sight.

However, epidemiologists and researchers are also casting doubt on the results of this antibody testing. Some of the most reputable medical journals in the world have criticized the results of the antibody testing conducted in Santa Clara County and other regions as lacking scientific credibility.

One area of criticism has focused on the sampling method. The antibody testing in Santa Clara County used SNS to solicit requests for testing. It has been noted the method was thus not random sampling, but possibly resulted in a higher rate of those who suspected that they might be infected.

In addition to the sampling method, there are problems with the method of antibody testing itself. The testing method only detects the percentage of response to COVID-19 and that is said to be its specificity. However, while usual antibody testing methods have a specificity of about 99%, this recent antibody testing used a test kit having only about 90% specificity. As a result, it has been criticized as having detected past viral infections (for example, infection by older coronaviruses) and then judged them to be a COVID-19 infection.

This explains why a major Swiss pharmaceutical company that has developed and sold antibody testing methods for different viruses over the years has made an unprecedented press release, announcing that the various antibody testing methods for SARS-CoV-2 (new coronavirus) in the past are not reliable.

I believe that this problem will be resolved soon, but rather than hesitating over numbers, it is first important to avoid COVID-19 infection by making sure to wash our hands and refrain from going out unless it is absolutely essential and urgent.