In recent weeks, as cases of the delta variant have increased dramatically in the United States, there’s been talk about “breakthrough” COVID-19 cases but not as much discussion about what these cases actually mean.
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To get to the bottom of what COVID-19 breakthrough cases actually are, how many breakthrough cases there are and what this means for the COVID-19 vaccines, we talked to infectious disease expert Steven Gordon, MD.
What is a breakthrough COVID-19 case?
According to the CDC, a “breakthrough” case is when a person tests positive for COVID-19 at least two weeks after becoming fully vaccinated (either receiving a one-dose vaccine or the second dose of a two-shot vaccine). This is, as the CDC also notes, to be expected. While the COVID-19 vaccines are highly effective at delivering immunity, no vaccine is 100% effective.
But another goal of vaccination is preventing serious illness. And in that regard, the COVID-19 vaccines are successful.
How many breakthrough cases are there?
While it’s still possible for those who receive the vaccine to contract COVID-19, it’s important to remember that those numbers are low. More importantly, the number of breakthrough cases that result in serious illnesses, hospitalizations or death are extremely low.
“The numbers are so low that the CDC is now primarily focusing on those breakthrough cases that result in hospitalization or death,” says Dr. Gordon. “And that number is incredibly small. Out of 160 million Americans who were fully vaccinated by mid-July, there have been less than 6,000 of those cases,” Dr. Gordon says.
To put that in perspective, that’s only 0.0037% of vaccinated Americans. Most of those cases (74%) occurred in patients 65 years old or older, but breakthrough cases can occur with a patient of any age.
Is the delta variant causing breakthrough cases?
While most current breakthrough cases are from the delta variant, it’s important to note breakthrough cases can come from all COVID-19 variants.
“The delta variant makes up the majority of breakthrough cases in the United States right now because it’s the dominant strain in the country,” Dr. Gordon points out. According to the CDC, the Delta variant made up only 10% of U.S. cases at the beginning of June. But by mid-July, just six weeks later, the Delta variant accounted for over 83% of U.S. cases.
This is because the delta variant is far more transmissible than previous variants of the COVID-19 virus, according to preliminary studies. One study out of China claims the delta variant viral load is 1,000 times higher than the initial strains at the beginning of the pandemic. The higher the viral load, the more likely the person carrying the virus is to spread it to others.
Underscoring the highly contagious nature of the delta variant is a UK study on household transmissions that found a 64% increase in the likelihood of infections compared to the alpha variant (also known as the UK variant which itself was more transmissible than the original variants).
Even though the delta variant is more transmissible, though, Dr. Gordon notes, “At this point, cases from the delta variant haven’t proven to cause more serious illness than other variants.”
The importance of getting the COVID-19 vaccine
The highly transmissible nature of the delta variant proves the importance of getting fully vaccinated against COVID-19. “The vaccines that are available here in the United States are effective against all variants of concern to date, including the delta variant,” says Dr. Gordon.
That means getting both doses of the two-shot mRNA vaccines (Pfizer and Moderna) so you’re fully protected. While studies cited by the CDC showed the effectiveness of just one shot of Pfizer was between 60% and 80%, getting both shots boosted that rate of protection to over 90%. The Moderna vaccine had a similarly high rate above 90% with both doses.
“Even with those variants of concern, the most important tool we have for prevention of getting an infection is getting vaccinated,” Dr. Gordon says.
“Throughout the pandemic, we’ve talked about flattening the curve so we don’t overwhelm the healthcare system with sick people,” he adds. “And the vaccine has effectively done that. These recent spikes are among the unvaccinated, not among people who are fully vaccinated.”
Even if you’ve received the vaccine and still don’t feel particularly safe or live in an area with a high rate of cases, Dr. Gordon says the best course of action is to keep wearing masks. “You can cut down on those risk factors by wearing masks, especially indoors with other people around,” he says. “But the bottom line is still to make sure, first and foremost, you get fully vaccinated.”