How COVID-19 tests pin down diagnoses

The most widely used test to diagnose COVID-19 right now is a molecular test that looks for genetic material from the virus, according to the Chester County Health Department, which is assisting Delaware County during the crisis. Antibody tests are also starting to become available, but because it can take several days for the body to begin producing antibodies to the virus, they are not recommended for initial COVID-19 diagnoses.

Molecular tests can have up to a 30% false negative rate, according to the FAQ, and are only searching for the presence of RNA, meaning they could detect dead virus material in a person who is no longer contagious.

The antibody test being used by the Chester County department is fairly accurate but is not useful within the first few days of exposure. It looks for two specific antibodies called “IgM” and “IgG.” The former is often the first antibody made when a person is exposed to a germ, while the latter is made after about a week of illness and tends to linger.

The department indicates people with IgG have likely had COVID-19 and are in recovery, or have recovered, and may have immunity to COVID-19. But it is unknown at this point whether people who had COVID-19 in the past can get infected again or how long immunity might last. It is also unknown if people who are immune can give the virus to other people who are not immune, according to the department, even if they don’t get sick again themselves.

Chester County’s list of frequently asked questions also discusses the concept of “herd immunity,” in which a population that has achieved 60 percent to 80 percent immunity to a virus helps protect non-immune members because the transmission rate is slowed or effectively halted.

Unfortunately, nowhere in the world is close to this threshold right now – less than 5 percent of the population has immunity and it could be as low as 1 percent or 2 percent, according to the FAQ – and the department says attempting to “force” herd immunity by ignoring precautions like masks and stay-at-home orders could result in an explosion of cases that would overwhelm an already strained healthcare system.