What to Know About Breakthrough COVID-19 Infections

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Mini Kamboj, MSK's Chief Medical Epidemiologist

Mini Kamboj, MSK's Chief Medical Epidemiologist

You may have read in the news about breakthrough COVID-19 infections, which are a rare occurrence in people who have been fully vaccinated. Memorial Sloan Kettering Cancer Center Chief Medical Epidemiologist and infectious disease expert Mini Kamboj explains that when these breakthrough infections do occur, they are not nearly as serious as COVID-19 infections in people who are unvaccinated.

“Even with the emergence of the Delta variant, all the data we are seeing shows that the vaccines work and achieve the most important goal: preventing hospitalizations and deaths from COVID-19,” says Dr. Kamboj. Here, she answers common questions about breakthrough infections and what they mean for MSK patients and staff.

What is a breakthrough COVID-19 infection?

A breakthrough infection means someone tests positive for the virus more than 14 days after they have been fully vaccinated. That means two weeks after the second shot of the Moderna COVID-19 vaccine or Pfizer-BioNTech COVID-19 vaccine (Comirnaty®) or two weeks after the one-shot Johnson & Johnson COVID-19 vaccine. No vaccine is 100% effective at preventing infection, so it has not been surprising that a small number of vaccinated people are still getting infected with COVID-19.

A breakthrough infection means someone tests positive for the virus more than 14 days after they have been fully vaccinated.
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How common are breakthrough infections?

Breakthrough COVD-19 infections are uncommon. The Kaiser Family Foundation (KFF), a nonprofit organization focusing on national health issues, analyzed data from the 24 states that have reported breakthrough infections and found that as of July 29, 2021, the rate of such cases among fully vaccinated people was well below 1%.

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How serious are the symptoms from breakthrough infections?

Up to a quarter of individuals with breakthrough infections may have no symptoms and not even be aware they have the virus. For those who do feel sick, the symptoms are usually mild and last a few days to one or two weeks. These symptoms typically include headache, cough, fever, and fatigue. Serious symptoms are unusual. The KFF found that the hospitalization rate for fully vaccinated people is 0.6% among the states analyzed. According to the CDC, the death rate is even lower — less than .001%. Among those who died, the cause of death may or may not have been COVID-19. Of course, the CDC still takes these cases seriously and is looking into them.

Breakthrough infections from the newer Delta variant of the COVID-19 virus are also mild. In a recent CDC report of the Delta variant outbreak in Barnstable County, Massachusetts, only four (1.2%) fully vaccinated persons were hospitalized. Two of the four had underlying health conditions, and no deaths were reported. Although the risk of breakthrough infection with Delta is higher than previous virus strains, the vaccine protection from severe disease remains above 90%.  

New Information: What We Know about Omicron and Other COVID-19 Variants
You may have heard about a new variant of COVID-19 called Omicron. There are many questions about Omicron, and scientists will have more answers in the coming weeks.
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Who is most at risk for breakthrough infections?

The elderly and those with compromised immune systems are most at risk. Aging impairs the ability of the immune system to protect against infection. The same is true for people who are immunocompromised. This can include some cancer patients, especially those who have had a bone marrow transplant. The CDC is actively considering a third vaccine dose for immunocompromised patients, and MSK experts are closely following these developments.

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How many breakthrough cases have occurred among MSK staff?

Since January 2021, only 146 breakthrough infections have occurred in MSK staff, out of a total of about 19,000 fully vaccinated employees. That is well below 1%. None of these MSK employees had to be hospitalized. Among vaccinated MSK cancer patients, hospitalizations due to COVID-19 continue to be low despite the rise in community rates due to the Delta variant. This shows that the vaccines are also working to prevent severe illness among our patients. MSK policy requires wearing masks in all clinical areas and public spaces where patients may be present, whether or not someone is vaccinated.

MSK continues to strongly recommend vaccination for all patients and their caregivers and has created a system for scheduling a vaccination appointment. We also have strongly urged all employees to get vaccinated. Since early summer 2021, MSK has required that employees who are fully vaccinated to wear a COVID-19 vaccination sticker on their MSK identification badge.

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Will the COVID-19 vaccines protect against breakthrough infection?

The COVID-19 vaccines now available were created for the original strain of the virus. The Delta variant, which now accounts for more than 80% of new COVID-19 cases in the US, is more contagious than the original strain. Although the vaccine effectiveness against the Delta variant infection is slightly lower, all COVID-19 vaccines remain highly protective (more than 90% effective) against severe illness and hospitalization, including from the Delta variant.

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Can people with breakthrough infections transmit the virus to others?

Emerging evidence suggests that vaccinated people may be less contagious because they have milder illness and shed the virus for a shorter time, but the extent of this difference between vaccinated and unvaccinated people is not definitively known. Until more studies are available, it is wise for everyone – including vaccinated people — to mask up indoors, especially with the Delta variant becoming so dominant.


August 11, 2021


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