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In a rare yet expected development, a small number of people have developed COVID-19 after they have been vaccinated.
"Nothing is 100% in this world," said Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security.
As new variants emerge around the world, however, scientists say it is critical to look out for these rare cases. They could be signs of something more serious.
"It's possible that more variants might emerge that are not covered by the vaccine," Gronvall said. "That's why it's important that these breakthrough cases are investigated."
Of the 95 million people fully vaccinated by April 26, 9,245 COVID-19 infections were reported to the U.S. Centers for Disease Control and Prevention (CDC). That comes to less than one-hundredth of a percent, though CDC says it is an undercount.
"Not every single individual that receives the vaccine is going to respond" to it, said Jacques Ravel, associate director for genomics at the University of Maryland Institute for Genome Sciences. "We are not 100 percent sure that even though those people have been vaccinated, they actually had mounted an immune response."
To discern the difference between a weak immune response and a breakthrough virus variant, scientists need to read the genetic code, or sequence, of the virus causing the vaccinated patient's infection.
The University of Maryland is part of a nationwide network of institutions sequencing COVID-19 coronaviruses. Ravel said his institute has not seen any patterns in the viruses infecting vaccinated patients. This, he said, more likely indicates "people who did not respond positively to the vaccine and are just infected because they were not protected."
If, on the other hand, "you start seeing over and over the same variant in that population, that's when you start to be worried," he said.
So far, the vaccines generally are working extremely well, even against the variants. In the latest real-world findings from Israel, where the variant first identified in Britain is dominant, the Pfizer-BioNTech vaccine was 95% effective. The U.K. variant spreads more easily and may be deadlier than the original.
But there are some cautionary signs. In a study in Qatar, where the variant that emerged in South Africa made up half the cases, the Pfizer-BioNTech vaccine was only 75% effective. That variant has also proven to be better able to break through other vaccines as well.
The shots still protect against the worst cases, though. The Pfizer-BioNTech vaccine was more than 97% effective in preventing severe illness and death in the Qatari study, for example.
"Even if you have a breakthrough infection," Gronvall said, the vaccine "is still really worth it."
The "good" news, if you can call it that, is that the British variant is so transmissible that it is crowding out other variants in many parts of the world.
"It's almost - I would not say a positive thing, but it's really helping in taming the more dangerous variants," Ravel said.
In another piece of dubious good news, many of the most concerning variants share the same mutations, even when they appear on different continents. That may mean that the virus can mutate in only a limited number of ways.
"Maybe this is it," Gronvall said. "Maybe it's not possible for the virus to have entirely different mutations that we haven't seen yet and still function."
Vaccine makers are already adapting their shots to the new strains, just in case. They are not needed yet, but new variants continue to arise.
"We are in a race," Ravel said. "We're fighting this battle constantly, and we just can't put our guard down."