With one in four adults in the United States now fully vaccinated against COVID-19, health departments and scientific studies have documented what are known as ‘breakthrough’ cases, or people who became infected or became ill despite being vaccinated.
Such cases are extremely rare, but are expected, experts say.
“You will see breakthrough infections in any vaccination when you literally vaccinate tens and tens and tens of millions of people,” said Dr. Anthony Fauci said during a press release from the White House on March 26. “In some ways, it’s not surprising,” Fauci said.
COVID-19 vaccines are very effective – but none are 100% effective, which means that a small number of people can become infected after being fully vaccinated – but the infections are usually mild or asymptomatic.
Dr William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, called the efficacy rate of about 95% for the Pfizer and Modern vaccines “spectacularly effective.” In clinical trials, the vaccines were almost 100% effective in keeping people out of the hospital, he added, pointing out that “there were still a few milder infections.”
How common are COVID-19 ‘breakthrough’ cases?
“It’s pretty rare,” said Dr. Shira Abeles, a specialist in infectious diseases at UC San Diego Health, spoke about breakthrough cases. “I think it will stay that way,” she adds.
Recent actual studies have confirmed the low breakthrough rate. Research that helped Abeles in Southern California between December 2020 and February 2021 found that less than 1% of the 36,600 health workers studied tested positive for COVID-19 after being fully vaccinated, meaning that both doses plus two weeks to build immunity – with the Pfizer or Moderna vaccines.
None of the vaccinated health workers who contracted COVID-19 were admitted to hospital or died, and those who did become ill, according to the researchers, had milder symptoms than those who were not vaccinated.
An even larger study conducted by the Washington State Department of Health found that there were 1.2 million continuous cases of 1.2 million fully vaccinated, which is less than 0.01% of the population living in fully vaccinated, represented. While most of the patients experienced mild symptoms, at least eight were admitted to the hospital and the health department is investigating two possible breakthrough cases where the patients died.
How do COVID-19 breakthroughs occur?
This is not entirely clear.
People who are very old or with an immune weakening are at a higher risk for breakthrough cases because their bodies may produce a less potent immune response to the vaccine.
“For every vaccine we use, people with immune use and the elderly do not respond as well as healthy people and younger people,” Schaffner said. In the case of the Washington state investigation into the two possible breakthroughs of COVID deaths, both patients were over 80 years old and had underlying health problems.
Similar to flu, the higher risk for older and immune users is one of the reasons why it is important for young, healthy people to get the COVID-19 vaccine. If everyone is vaccinated, it is more difficult for the virus to find targets, which protects those at higher risk.
“We all need to be vaccinated to protect ourselves as well as to protect the weaker ones among us,” Schaffner said.
As for breakthrough cases among young or healthy people, “every now and then something happens for biological reasons that we can not completely explain,” Schaffner said.
Those who do get sick after being completely vaccinated still need to be partially protected. Instead of going to the hospital or dying, they may have less severe symptoms, such as feeling lethargic or having a mild fever. “This is the strength of our study,” Abeles notes. “We did a survey among a population, and we picked up asymptomatic diseases and mild diseases,” she said. “That’s generally what you’re going to see.”
Can new variants cause ‘breakthrough’ cases?
Another possibility that public health officials are investigating is whether new variants of the virus have caused some of the reported breakthrough cases.
“One of the important things that has been done and needs to be done is to sequence the genome of the virus that is the breakthrough virus,” Fauci said. ‘It’s very important to see if they break through with the wild-type virus, which indicates a real reduction in immunity, or if it breaks through with one of the variants, which would be much more explicable if you did not do so. . have sufficient cross-reactivity. ‘
So far, it seems that the currently allowed vaccines reduce the severity of infection, regardless of the virus strain. This is why experts believe that it is of utmost importance that even people previously infected with COVID be vaccinated. To know if breakthrough cases were caused by the virus, and that the vaccines are designed to protect against or by new variants, scientists can inform how to approach future vaccinations and shots. If a variety of concerns breaks through, scientists in the laboratory can develop a vaccine that targets the protein mutations of the variant, Abeles explains. If the virus is one to which the community has already been exposed, and immunity seems to be declining, health experts could give a boost to the same vaccine, she added.
“It is a gripping target and we are still learning so much,” Abeles said. “We will pay close attention.”
Meanwhile, it is important to keep infection levels low so that there is less evolutionary pressure on the virus to turn into more variants of concern. “We want to keep it on a few, rather than let the virus get too many opportunities,” Abeles said.
ABC News’s Morgan Winsor contributed to this report.