Healthy Tahoe: How COVID vaccine prevents illness

Rhonda Sneeringer, MD

Some are wondering about the vaccine’s effectiveness against the COVID-19 virus. With the emergence of the delta variant, many people may know of a vaccinated person who’s been infected with COVID-19 after vaccination.

It can be confusing and even alarming, especially with the high risk associated with contracting the virus. But rest assured, the vaccine is effective and it is protecting us.

Dr. Rhonda Sneeringer

So what happens when you are exposed to the SARS-CoV-2 virus as a vaccinated person? First, you are exposed to somebody who’s COVID-19 positive. The virus enters your nose, and the spike proteins on the virus, which are unique to COVID-19, try to find your ACE2 receptors — a protein on the surface of your cells that acts as a cellular doorway — to bind to and enter your cells to start replicating.

Since you’re a vaccinated person, you have antibodies — protective proteins produced by the body’s immune system — floating around, specifically built to recognize the COVID-19 spike proteins. Those circulating antibodies bind to the spike proteins, effectively neutralizing and stopping them from entering your cells.

Depending on the amount of virus you’re exposed to, the amount of circulating antibodies you have from the vaccine may not be enough to neutralize every single spike protein that enters your body. For instance, the antibodies may neutralize 85% of all the spike proteins that enter, but 15% of the spike proteins (and virus) still enter your cells, start replicating, and then go on to infect other cells, which can make you mildly sick.

After your body’s initial antibody response, your secondary immune response includes a memory playbook where your B cells quickly make more antibodies and cytotoxic T cells find infected cells and kill them.

Since your body already has this memory playbook that kicks in quickly, you’ll experience fewer and milder symptoms, the duration of your symptoms will be much shorter, and your body is going to clear the virus faster. You are significantly less likely to progress to severe illness, hospitalization, or death. Vaccines work by giving your body a head start and a fighting chance.

On the other hand, if you’re unvaccinated and get exposed to COVID-19, you don’t have that head start. You don’t have those circulating antibodies already there to help neutralize the virus right away. There’s no pre-existing antibody memory that starts fighting sooner. More of the virus gets into your cells quicker than if your body already knew how to fight it. You’ll have worse symptoms and longer duration of illness, because this is the first time your body has been exposed. For some people, their bodies get overwhelmed with the amount of virus resulting in severe illness, hospitalization and sometimes death.

If you already had COVID-19, you may ask yourself why you need a vaccine. While we are still learning about the immunity of those who have had COVID-19, we know the protection is not as strong as those who have been vaccinated. Early information indicates it may take multiple infections to equal the immunity of the vaccine.

Your immune system is designed to work against the virus, but the vaccine is the best way to ensure it gets a head start and maximizes protection. If you have the vaccine, your body will know what to do as soon as it is exposed, which will help you beat the virus before it grows too quickly.

Dr. Rhonda Sneeringer is the Medical Director of Outpatient COVID-19 Care at Barton Health. To learn more about the COVID-19 vaccine or what to do if you’re feeling symptoms or have been exposed, visit

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