At the start of the summer, it looked like COVID-19 was in retreat in the United States. Daily new cases plummeted from more than 300,000 at their peak in January to around 10,000 in June — a reduction of more than 95 percent. The decline was thanks in large part to rising vaccination rates across the country.
But in early July, COVID-19 case numbers began to climb once more.
The surge in cases prompted the U.S. Center for Disease Control and Prevention (CDC) to update its mask guidelines July 27, returning to an older recommendation that everyone — including the vaccinated — should mask up in enclosed public spaces. For many, this turn-around might feel disheartening or even confusing.
The delta variant, also known as B.1.617.2, is a strain of SARS-CoV-2 virus first identified in India in December 2020. It has since become widespread, with cases reported in over 100 countries worldwide. In the U.S., it now accounts for more than 80 percent of new COVID-19 infections, according to a separate CDC report.
For months there was uncertainty surrounding the delta variant; for example, there had been debate on whether it caused more severe symptoms than the wild strain. "Some estimates have put the delta variant at maybe two, three or four times more transmissible than the original virus," says Ben Cowling, an epidemiologist at the University of Hong Kong.
But the new data from the CDC confirms it's more easily spread. An internal CDC slide presentation, which was leaked to The Washington Post, says every person infected with the delta variant will, on average, infect eight or nine more people. This is what's known as its basic reproduction number, or R0 (pronounced R-naught).
"When you think about diseases that have an R0 of eight or nine, there aren't that many," CDC Director Dr. Rochelle Walensky told CNN Friday. She also confirmed the authenticity of The Post document.
Another reason for the high R0 is delta's viral load. Compared with the wild strain of the virus, a study published July 7 in Genomic Epidemiology found that delta generates over 1,000 times more viral particles in a patient's body before they begin showing symptoms. That means the incubation process for delta is quicker, and the time for someone to become contagious is quicker, Cowling says.
"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus," Walensky said in a statement Friday.
So far, vaccines seem to be effective in fighting off earlier strains of coronavirus, but the delta strain is different. The CDC data show that vaccines prevent more than 90 percent of severe disease, but they are less effective at preventing overall infection and transmission from delta. That means, according to the new data, we should expect more breakthrough infections in vaccinated individuals. And while these breakthrough infections are milder compared to cases unvaccinated folks will see, it does mean fully vaccinated people with breakthrough infections can spread delta in their communities as fast and easily as those who are not vaccinated.
That's where the new mask recommendations come in.
Masks Up, COVID-19 Down
The new data reiterate vaccinations are by far the most effective way to prevent severe illness and death from COVID-19. Between 97 percent and 99 percent of Americans currently hospitalized with severe COVID-19 are those who are unvaccinated.
But masks are also effective at preventing the spread of COVID-19 when you wear them properly. "The key is making sure that the mask has a good fit," says Dr. Nathaniel Beers, a pediatrician at Children's National Hospital. A snug mask is a good mask; it should still allow you to breathe but be fitted and layered enough to stop respiratory droplets from flying. This type of masking can reduce COVID-19 transmission by 65 percent to 80 percent, depending on the fabric. "It's not perfect, but it can have a really substantial effect," Cowling says.
And masks don't just work against COVID-19. There is good evidence that widespread mask use reduces flu transmission, and epidemiologists think that masking during a flight may have helped prevent an outbreak of monkeypox in the U.S. earlier this month.
Your Guide to the Guidelines
So, what do the CDC's mask guidelines actually mean?
One space where masking remains an especially sticky issue, though, is in schools. Since children under age 12 do not have the option to be vaccinated, some parents and teachers worry that schools will become hot spots for COVID-19 transmission when they reopen this fall. And nobody is eager to return to online classes.
Like the CDC, the American Academy of Pediatrics (AAP) recently released its COVID-19 guidelines for the 2021-2022 academic year. In it, the group recommended masking for all school students and staff to help prevent the spread of the delta variant.
"Rather than having schools try and figure out who is and isn't vaccinated," says Beers, who is the former president of the AAP's D.C. Chapter, "we felt it was appropriate to make the recommendation that all students and staff continue to use masking."
Though masking all students may sound like a drastic measure, Beers and the AAP stress the importance of safely returning to in-person, rather than remote, learning.
It should be noted that organizations like the CDC and the AAP cannot enact policy — they can only advise lawmakers based on their body of research. It's up to federal and state agencies, local governments, and even individual businesses to decide when and how to implement things like mask mandates.
Cowling thinks that partial masking might be the way to go moving forward. In Hong Kong, it is standard practice for sick students to wear a mask during class in order to protect their fellow students. This has been the norm since well before COVID-19.
Ultimately, though, masks are meant to be a temporary measure. "I think that vaccines should be considered more seriously for children," Cowling says. Not only would this allow students to go safely barefaced, it would also offer some level of resistance to new variants when they inevitably arise.
"I don't think COVID is going to go away," Cowling says, "so there's a clear choice: Would you rather children have immunity from infection or immunity from vaccination?"
Now That's Interesting
There are four main types of flu virus: A, B, C and D. However, there are many variants of each type of flu — including some that haven't evolved yet. For example, while we know that there are 198 possible variants of influenza A, we've only observed 131 in nature.
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