COVID-19 Breakthrough Infections: 7 Facts to Know

While cases of COVID-19 after vaccination are uncommon, an expert explains how to protect yourself and your loved ones given the rapid spread of the Delta variant.

The spread of the Delta variant has led to a rise in COVID-19 cases in the United States, including among those who are fully vaccinated. So what’s important to know about these breakthrough infections?

Breakthrough infections — cases among those who are fully vaccinated against COVID-19 — account for well below 1 percent of COVID cases in all reporting states, according to a Kaiser Family Foundation analysis of the state-reported data. As of August 2, more than 164 million people in the United States have been fully vaccinated, and the Centers for Disease Control and Prevention (CDC) has received reports of 7,525 breakthrough infections that have led to hospitalizations or deaths, which represents 0.004% of the vaccinated population.

“The majority of new cases are among those who are unvaccinated, and notably, almost all deaths due to COVID-19 are among people who are unvaccinated,” says Dr. Sharon M. Chacko, medical director at the Farrell Community Health Center at NewYork-Presbyterian’s Ambulatory Care Network and medical director of COVID-19 immunizations in the Division of Community and Population Health.

Dr. Sharon Chacko

Dr. Sharon Chacko

While breakthrough infections are uncommon, new CDC research found that vaccinated people with mild or even asymptomatic cases of COVID-19 can spread it to others. In a study of an outbreak in Provincetown, Massachusetts in early to mid-July — a period in which the town held multiple summer events and large public gatherings — the CDC found that 74% of the cases occurred in fully vaccinated people. The Delta variant was detected in 90% of these cases. In the wake of the rise of the Delta variant and research into breakthrough infections, the CDC now recommends that vaccinated people wear masks in public indoor settings in communities with high or substantial transmission of COVID-19.

“Of the breakthrough infections in Provincetown, seven people were hospitalized and zero died. If you compare this to the rate of hospitalization and death during earlier outbreaks prior to vaccine availability or outbreaks among unvaccinated individuals, there is an incredible difference,” explains Dr. Chacko. “What is critical is the efficacy of the vaccines in preventing severe COVID. Ultimately, the measure of the success of any vaccine is its ability to prevent significant illness, hospitalization, and death. Vaccines reduce your risk of hospitalization from COVID by 25-fold and risk of death from COVID by 25-fold.”

Dr. Chacko, ​​who is also an assistant professor of medicine at Columbia University Vagelos College of Physicians and Surgeons, shares what experts currently know about breakthrough infections. Here are seven key facts:

1. A breakthrough infection is an infection that occurs at least 14 days after being fully vaccinated.
A breakthrough case is confirmed with a respiratory specimen. If SARS-CoV-2 RNA or antigen is detected 14 or more days after full vaccination (two doses of the Pfizer or Moderna vaccine, or one dose of the Johnson & Johnson vaccine), that is considered a breakthrough case. This is because it typically takes 14 days after the full vaccination series for the body to build immunity against the SARS-CoV-2 virus that causes COVID-19. “After vaccination it is currently unlikely, but possible, that one could develop a breakthrough infection,” says Dr. Chacko.

2. A certain number of breakthrough infections were expected to occur.
While the efficacy is very high for all three vaccines — Pfizer, Moderna, and Johnson & Johnson — it’s not surprising to see some people experience breakthrough infections since no vaccine is 100% effective. “As more people are vaccinated, the number of cases that are considered ‘breakthroughs’ will rise simply because of the math — more people are vaccinated than unvaccinated,” says Dr. Chacko.

3. Breakthrough infections are typically asymptomatic or mild.
Of the breakthrough cases seen, symptoms are extremely likely to be mild to non-existent for people who have been vaccinated. “One of the most important reasons to get vaccinated is to prevent severe COVID-19 infections, and all three COVID-19 vaccines in use in the U.S. demonstrate extremely high efficacy against severe COVID-19 infections that result in hospitalization or death,” says Dr. Chacko.

“Of the breakthrough cases seen, symptoms are extremely likely to be mild to non-existent for people who have been vaccinated.”

4. The Delta variant is responsible for the majority of breakthrough infections in the U.S.
To date, the Delta variant is the most transmissible strain of SARS-CoV-2 and accounts for about 83% of all COVID-19 cases in the U.S. According to the CDC, it is more transmissible than influenza and the virus that causes Ebola. Data show that the virus stays in the mucosal lining of the nose for much longer than previous variants. “People who are infected with the Delta variant have higher viral loads that they can spread to others,” says Dr. Chacko.

Because of this high transmissibility, the Delta variant has caused an extremely rapid rise in cases. Areas with lower vaccination rates are disproportionately experiencing the highest case rates, and almost all cases of hospitalization and death are among those who are unvaccinated.

5. Wearing a mask can help protect yourself and others from breakthrough infections.
The CDC recommends wearing a mask in public indoor settings in areas with substantial transmission, which includes New York City. This is a way to protect against a breakthrough infection and to prevent transmitting the virus to another person.

“At this time, masks provide another layer of protection against COVID,” says Dr. Chacko. “This protection is especially critical for those who may be at higher risk if exposed to COVID and for those who have contact with others who may be at higher risk, such as someone who is immunocompromised or a child who is not yet eligible for vaccination. Some have asked why the CDC updated its recommendation. It is imperative that guidelines reflect the latest data and respond to the evolving situation, which means there will be frequent updates.”

6. Vaccinated individuals who were exposed to COVID-19 or have symptoms should get tested and isolate if positive.
COVID-19 symptoms can include fever, chills, cough, difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, runny nose, nausea, vomiting, and diarrhea. With other cold viruses also circulating, it’s important to get tested to know if any symptoms are from a COVID-19 infection. If your result is positive, the CDC recommends isolating for 10 days.

“If you are fully vaccinated and exposed to someone with COVID-19, the CDC recommends that you get tested three to five days after the date of exposure and wear a mask in public indoor settings for 14 days after exposure or until a negative test result,” says Dr. Chacko. “Of course, you should still follow your local and workplace regulations including contacting your employer for additional guidance if applicable.”

7. Vaccination is still our best defense against COVID-19.
According to the CDC, more than 97% of those hospitalized with COVID-19 are unvaccinated. The bottom line is that vaccines save lives. “Vaccines are your best tool to protect yourself and your loved ones,” says Dr. Chacko. “On a larger scale, vaccines are key to ending the COVID-19 pandemic.”

Sharon M. Chacko, M.D., is a family medicine physician and the medical director of the Herman “Denny” Farrell, Jr. Community Health Center at NewYork-Presbyterian’s Ambulatory Care Network and an assistant professor of medicine in the Center for Family and Community Medicine at Columbia University Vagelos College of Physicians and Surgeons. Dr. Chacko has been a medical lead for NewYork-Presbyterian’s COVID-19 vaccination efforts and served as medical director at the vaccination site established by NewYork-Presbyterian at The Armory. She is now medical director of COVID-19 immunizations in the Division of Community and Population Health.