COVID-19 Basic Immune Response

Just for clarification: SARS-2 is the virus that causes COVID-19. The current COVID-19 PCR test measures viral RNA. It does not measure our immune response to the virus.

Viral RNA is only found in blood with active viral infection (or for a short period of time after recovery). In fact, in theory, we should get two consecutive negative PCR tests before considering someone previously infected “non-infectious”. When someone recovers from COVID-19, the PCR shortly after becomes negative.

To determine past exposure to SARS-2 we use serologic tests, which measure our immune response to the virus. These tests, which require blood (serum) collection, are able to quantify specific IgM and IgG levels against SARS-2. So, SARS-2 IgM and IgG are antibodies that infected people produce during the course of the infection (Ig M first, followed by Ig G). These antibodies normally (in most other viral infections) are long-lasting, and could protect the body from future infection with the same virus.

Most anti-viral antibodies are protective, for example, hepatitis B IgG (or HBS Ab), which protects the body from HBV re-infection. Sometimes antibodies that the body produces after a viral infection can last for life but are not protective, such as the ones our body produces after a hepatitis C infection (thus we can get re-infected with HCV).

Currently, we can measure SARS-2 antibodies, but their utility to help in the diagnosis of acute infection or recovery are not better than the PCR. They can be used for epidemiological reasons (to find out the proportion of people already infected or susceptible).

They could potentially be used for treatment, using concentrated serum from people who recovered from the infection, to treat the acutely infected person. Think about it like the immunoglobulin shot some people get during an outbreak of hepatitis A. The validity of this approach for treatment of COVID-19 has not been well studied.

The ability of any anti-viral antibodies to protect a person later depends on various factors, such as:

The antibody preferably needs to attach to a crucial viral part, making it unable to replicate. Sometimes the antibodies the body produces attach to a virus part that do not neutralize the virus.
Our body needs to create a memory T-cell response that will recognize the virus later, after a future re-exposure.
The viral surface glycoproteins, which are the sites where the antibodies attach, should not change much with time. In essence (although not quite), this is what happens after infection with the influenza virus.

It is unclear how long-lasting the antibodies produced by the body during COVID-19 are able to last or to protect the body in the future. There has been some case-reports of re-infection, which if true, it suggests that our body’s antibody production may not be fully protective after COVID-19.In reality, it is my understanding that this is yet to be elucidated.

If COVID-19 becomes a recurrent infection, SARS-2 serology tests may play a clinical role in the future. For the time being, we just don’t know yet.

By Edwin DeJesus, MD, FACP, FIDSA

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