Can We Eliminate COVID?

It’s important to understand what COVID vaccines can and can’t do.

Some viral vaccines are so good that they can prevent even mild illness for decades and eliminate the virus from the face of the earth. Others are good at protecting against serious disease, but not as good at protecting against mild disease or spread. The difference has nothing to do with the intelligence of the scientists who invented the vaccines or the competence of the companies that made them. Rather, the effectiveness of vaccines is largely determined by one thing: the incubation period of the disease, which is the time between exposure to a virus and the onset of symptoms.

If incubation periods are long—meaning a couple of weeks, as for smallpox, measles, and German measles (rubella)—then the virus can be eliminated from the face of the earth. If incubation periods are short—meaning only a few days as for SARS-CoV-2, influenza, respiratory syncytial virus, and common cold viruses—then the virus is likely to circulate for centuries causing mild illness in most and severe disease in some.

Why do incubation periods determine the effectiveness of a vaccine?

The key determinant for protection against mild illness is the level of virus-specific antibodies present at the time of exposure to the virus. The good news is that these antibodies are readily induced by natural infection or vaccination. The bad news is that they don’t last very long—usually 3 to 6 months—before they fade away. Therefore, for diseases with short incubation periods, protection against mild illness is always short-lived. Always.

 SARS-CoV-2 vaccines are a perfect example of what you can expect from antibodies. When researchers from Pfizer and Moderna presented the results of their mRNA vaccine trials in December 2020, they showed that two doses induced 95 percent protection against both mild and severe disease. Why was protection against mild illness so high? The answer is that these studies were performed over a 3-month period; most of the participants had just received their second dose. For that reason, everyone still had high levels of antibodies in their bloodstream and, therefore, were protected against mild disease.

But protection against mild illness couldn’t last.

By the middle of 2021, about 6 months after people had received their second dose of vaccine—when antibodies had begun to decline—protection against mild illness had also declined from 95 percent to 50 percent.

Protection against severe illness, on the other hand, had remained high. That’s because protection against severe illness isn’t dependent on antibodies present at the time of exposure; it’s dependent on immune memory cells, like memory B cells, which can be stimulated to make antibodies. The good news about these immune memory cells is that they are long-lived, often for decades. The bad news is that it takes time after exposure to the virus for these memory cells to become activated and to make antibodies.

For diseases with short incubation periods, like COVID, symptoms begin before these memory B cells have had enough time to make antibodies. Therefore, people suffer mild illness. On the other hand, it takes much longer—about a couple of weeks—to develop severe COVID. That’s plenty of time for these memory B cells to become activated and to make antibodies to prevent severe disease. This is why protection against mild disease is short-lived and protection against severe disease is long-lived.

On the other hand, for diseases with long incubation periods, symptoms begin after memory B cells have had enough time to make antibodies that can prevent even mild illness and spread. This is why diseases with long incubation periods can be eliminated from the face of the earth.

Even if everyone in the world received a COVID vaccine, and SARS-CoV-2 virus didn’t form new variants, many people would still suffer mild disease and some severe disease every year. Like influenza virus, which has been around since the mid-1300s, it is unlikely that we will ever eliminate SARS-CoV-2 virus.