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Why are we seeing more vaccinated people becoming infected with the Delta variant?

Why are we seeing more vaccinated people becoming infected with the Delta variant? How protective are the vaccines at this point?

Both of those questions have been on a lot of people’s minds since the Delta variant took hold and more breakthrough cases began to appear. Almost all COVID cases in Maryland are now caused by Delta. This variant multiplies much more quickly and at higher concentrations in the respiratory tract than previous COVID strains. Previous strains usually took 5-6 days from the time of exposure until people began to feel sick. It’s more common for symptoms from Delta to begin 3-4 days after exposure to an infected person. Studies have detected 1,000 times more virus in the nose and upper respiratory tract in early Delta infections than with previous strains of COVID.

Our Health Department epidemiologists have reviewed all COVID cases during the past 2 weeks (the time at which Delta began to dominate) to determine the likelihood of infection in vaccinated compared to unvaccinated people. It is important to keep in mind that not everyone who becomes ill gets tested. If there are significant differences in testing rates between those who have and haven’t been vaccinated, it will impact the numbers that we are able to provide as well as statewide and national statistics.

Looking at close to 200 infections during the late July into early August, unvaccinated Calvert residents are 5.3-times more likely to get infected than vaccinated residents. Another way to express this is that vaccination is 81.2% protective against infection. Over these past 2 weeks, 6 Calvert residents have been hospitalized. All were unvaccinated. During that time period, fortunately, we have not had any deaths. National data shows 88% effectiveness of mRNA vaccines against COVID infection and a clear and dramatic reduction in hospitalizations (97-98% lower risk) and death. Studies from England indicate 80% effectiveness in preventing infection and 97-98% reduction in hospitalization, so our current local data is consistent with the expected results from U.S. and U.K. numbers.

The mRNA vaccines, which have been given to >90% of vaccinated Calvert residents, showed 93-94% effectiveness against previous strains of COVID. So why the 10-12% lower protection rate with the Delta variant? Higher viral counts are part of the answer, but if that was the only reason, we would expect even more infections and many more cases of severe illness leading to hospitalizations and deaths. The rest of the answer is found in how the body responds to a virus that it has previously encountered by exposure to an infected person or has been introduced to by a vaccine. The COVID vaccines don’t contain whole virus particles, but they do present the key COVID spike protein to our immune systems. (For an explanation of how mRNA vaccines work, you can read our previous post-

Our immune system is extremely complex and has multiple overlapping layers to keep us protected. The most important features of the immune system are the innate (always ready to go) immunity with components like “natural killer” cells and our adaptive (needs to be primed by a specific invader) immunity that produces neutralizing antibodies and programmed T cells. Our innate immunity is good at protecting against some infections, but its effectiveness is limited. Innate immunity is a relatively crude system that generates large inflammatory reactions in order to kill off invading viruses and bacteria. This inflammation ends up causing its own symptoms (congestion, muscle aches, etc.) as it generates an attack. Think of an army from centuries ago that launches an all-out direct attack on a battlefield hoping to overwhelm the enemy. The army may win, but there will almost certainly be damage on both sides. In the context of COVID, the inflammatory response in some people gets out of control and leads to dangerously reduced oxygen transfer through our lungs and life-threatening blood clots in our brains, kidneys, and other organs.

Our adaptive system is much more precise in its attack because it can specifically target unique features on the surface of a virus or bacterium that are not found on our own tissues. This results in little damage to our own bodies. The measles virus occasionally circulates, but if you’ve been vaccinated, you already have antibodies and programmed T cells that specifically target the virus. The antibodies and T cells silently and effectively destroy invading virus particles without needing to generate a huge inflammatory response. Please understand that there are many other components to both the innate and adaptive systems, but those are way beyond the scope of a webpost.

Neutralizing antibodies and programmed T cells are produced as a response to either natural infection or vaccination. It takes 1-2 weeks to produce this targeted protection the first time your body is exposed to a new virus or after the first dose of a vaccine. This is a key consideration because one unusual feature of COVID infections is the long delay between initial colonization of the virus in our upper respiratory tract (nose and upper airway that can cause coughing or bronchitis) and spread to our lower respiratory tract (lungs) and other organs.

Depending on many different factors that vary between individuals, including 1) the time between initial infection and full activation of the immune response; and 2) perhaps most importantly, the degree of inflammation their innate immune systems produce in response to the virus, an infection with COVID in an unvaccinated individual may result in anything from minimal symptoms to a life-threatening infection.

Multiple studies have proven that COVID vaccinations produce higher levels of neutralizing antibodies than natural infection. In addition, antibody levels last longer after vaccination, especially with the 2-dose Pfizer and Moderna vaccines. The importance of neutralizing antibodies is that they can almost instantly disable invading virus particles once they enter your body. In case you are wondering, the programmed T cells are harder to study and are not as well understood. There is preliminary evidence that T cell response is also stronger after vaccination than after a natural infection, but studies are not yet conclusive. With that understood, we can now answer the question as to why more vaccinated and previously infected individuals are contracting the Delta variant than was seen with earlier strains of COVID. The key is that neutralizing antibodies gradually decrease in your circulation over months. The duration that neutralizing antibodies actively circulate in our blood varies from person to person, so I cannot provide a precise duration.

But all is not lost. Queue the next paragraph….

When we are vaccinated or infected, we also produce immune system memory cells that last much longer than circulating antibodies. In response to a subsequent virus exposure, these memory cells will produce antibodies and programmed T cells much more quickly than if our bodies are starting from scratch. However, there is a delay of a few days for memory cells to fully crank into gear. As mentioned above, the Delta variant causes symptoms of infection about 2 days sooner than previous strains of COVID. As a result, some people whose memory cells would have responded quickly enough to prevent the initial symptoms of infection during earlier waves of COVID, are now seeing breakthrough infections. However, since it generally takes more than a week for a COVID infection to descend into our lungs and cause major damage, these breakthrough infections are almost always eliminated by your immune system before they can cause permanent scarring of your lungs, dangerous blood clots and strokes, or death.

I’ll wrap with two currently unanswerable questions. Why are people who have previously been infected with COVID more likely to develop severe infections when re-exposed months later to COVID than those who have been vaccinated? Although this is not fully understood, it may have to do with the lower antibody and memory cell levels seen in people after natural infection than after vaccination. Research has shown that people who receive vaccination after an infection have a major boost in circulating antibodies and a 5- to 10-fold increase in memory cells. That is why it’s important for previously infected people to get vaccinated. How long will vaccinations last? The answer hinges on the duration of memory cells in our bodies. This is an area of very active research. The first vaccinations were administered over a year ago. To this point, there is no indication of higher rates of serious COVID infections in those individuals, so we know that (barring a new variant that eludes current vaccines) protection lasts for at least a full year and may sustain for substantially longer. For individuals with certain immunocompromising conditions (suppression medication following an organ transplant, some forms of chemotherapy, etc.), studies are underway to see if additional booster doses are more protective. It’s also possible that elderly individuals and those with less severe forms of immunocompromise may benefit from boosters sooner than younger, healthier people. As research provides more clarity to these questions, we’ll keep you updated.

The bottom line is that vaccines continue to do a great job preventing infections and minimizing the consequences of illness from Delta. Safety data continues to show COVID vaccines are as safe or safer than any vaccines we’ve had in the past. Across Maryland, case rates have increased by 62% over the past week. The total number of hospitalizations in the past 7 days has increased from 222 admissions to 345, including 3 Calvert residents (all unvaccinated). Total ICU admissions have increased from 58 to 84. Based on previous waves of infections, this will get worse over the next two months. If you have previously chosen not to be vaccinated, now is the time. You can contact your personal doctor’s office or schedule an appointment through our health department at

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