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  • Writer's pictureWilliam Miller, MD

Breakthrough COVID: Are the Vaccines Failing to Work?

As the Delta variant fuels a surge in new cases, we are seeing some people get sick who were previously vaccinated. Does this mean the vaccines are not as effective as they once were?

Before we start, we should draw a few distinctions. First, there is a difference between getting infected and getting ill. Someone can be infected and without symptoms, yet still potentially contagious. An ill person is someone who is infected and has symptoms. We can further divide illness into mild, moderate or severe. With COVID, mild symptoms might include a dry cough, sore throat and headache, while moderate would include getting short of breath when walking across a room or feeling extremely fatigued. Severe symptoms would be having a very low oxygen level requiring treatment with oxygen, these people often require hospitalization. Failure of the lungs to the point of requiring a machine to assist with breathing would be critically ill and those people are in an ICU. Obviously, the next point in the continuum would be those who die of the disease.

Next, we need to consider what is meant when we say a vaccine is “effective”. The goal of a vaccine program is to reduce the infection rate in the population so that the disease does not spread as easily or quickly. This is by both stopping an infection from happening in the first place or if infection occurs anyway then reducing the viral load making the vaccinated person less contagious. Looking at how the disease is spreading in vaccinated versus unvaccinated populations is a way to judge whether a vaccine is “effective” or not. Another purpose of a vaccine is to reduce the severity of illness. Keeping a vaccinated person from progressing to serious illness or death would be considered good effectiveness of a vaccine.

On the other hand, a vaccine does not act as a barrier that will prevent all infections and all illness. No vaccine is 100% effective on all levels. The term “breakthrough” infection is perhaps not helpful as it seems to imply that somehow the virus “broke through” such a barrier. There will always be a small percentage of fully vaccinated people who get sick, are hospitalized, and even die from COVID-19. To judge the effectiveness of the vaccine we must compare the rates of serious illness and death in vaccinated versus unvaccinated people.

The vast majority of new cases across the US and locally are in unvaccinated people. According to the CDC, 99.6% of all new cases are in unvaccinated persons. The California Department of Public Health website indicates that for the week of August 9 – August 15, the average case rate among unvaccinated Californians was 51 per 100,000 per day while the average case rate among vaccinated Californians was significantly lower at 7.6 per 100,000 per day. They went on to state that there is a 500% higher risk of infection for unvaccinated persons right now compared with vaccinated persons. This is strong evidence that the vaccines do work in protecting against new infections even against Delta which now makes up well over 85% of all infections.

So, if this is true, then why are we still seeing cases? The reason is that if the vaccine protects against 92% of infections, that still means that 8% of people who are vaccinated can become infected if exposed. For example, if there is a large group of vaccinated people getting together with a large number of unvaccinated and potentially infected people, then you will see many vaccinated people coming down with COVID. This is the explanation of what happened in Barnstable County, Massachusetts, in July. Mask mandates and restrictions on public gatherings had been relaxed. During that month, there were several very large entertainment events bringing in thousands of participants and tourists. Despite having a large percentage of vaccinated people (69%) the community experienced an outbreak that included 346 vaccinated people. It is important to note, however, that only 4 of those vaccinated people required hospitalization and none died.

Perhaps most importantly, the vaccines remain effective in protecting against serious illness, hospitalization and death. A study of 7,280 vaccinated COVID patients was reported in the Morbidity and Mortality Weekly Report (MMWR) on August 13th stating “Among adults aged 65-75, effectiveness of full vaccination for preventing hospitalization was 96% for Pfizer-BioNTech, 96% for Moderna and 84% for Janssen COVID-19 vaccines; among adults aged >75 years, effectiveness of full vaccination for preventing hospitalization was 91% for Pfizer-BioNTech, 96% for Moderna and 85% for Janssen…” While this study is very helpful, there are two limitations. First, it looked at cases between February and April, 2021, which was prior to Delta becoming the predominant variant. Second, most of the people in the study had been recently vaccinated. Other studies have suggested that the rate of protecting against hospitalization with Delta is around 88% for Pfizer and Moderna as reported in the New England Journal of Medicine on July 21st.

There is evidence that the effectiveness of these vaccines does wane over time. A very robust study involving over 740,000 people in the United Kingdom was just published in the journal Nature on August 19th. Half of the study took place early in 2021 when the Alpha variant predominated and the other half was in the last three months when Delta predominated. Participants had all received either both doses of Pfizer or the single dose of AstraZeneca. It looked at the effectiveness of protecting a fully vaccinated person against infection by assessing viral load following a known exposure. The study found that the Pfizer vaccine “was 92% effective at keeping people from developing a high viral load… 14 days after the second dose. But the vaccine’s effectiveness fell to 90%, 85% and 78% after 30, 60 and 90 days, respectively.” Keep in mind that even at 78%, this is still considered good. This study only looked at viral load as an indicator that the person might be infectious but it did not look at whether or not the subjects developed symptoms or got hospitalized. However, it does support the notion that a booster shot at some point may be beneficial.

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