With surges in Covid-19 cases driven by the Delta variant, most Americans are now being advised to wear masks indoors -- regardless of vaccination status -- by the US Centers for Disease Control and Prevention.
Since CDC guidelines updated Tuesday, we have learned more about the science that motivated these changes, both from a leaked agency document and a study published Friday by the agency. The new research reinforces the ongoing need for everyone who can get vaccinated to get vaccinated.
Here are five key takeaways:
Delta is super transmissible
We knew Delta was spreading fast. In the United States and around the world, it swiftly overtook another variant, Alpha, that was already more transmissible than earlier strains.
New research from Helix, a company whose Covid-19 tests have helped track a number of variants, shows how quickly that happened: Alpha made up 67% of cases in mid-May. Ten weeks later, it was down to just 2.3% -- replaced by Delta, estimated around 90% of cases.
"It's one of the most transmissible viruses we know about. Measles, chickenpox, this -- they're all up there," Walensky told CNN Thursday.
In terms of how quickly they spread, early strains of the coronavirus were similar to the common cold, according to an internal CDC document. Left unchecked, an infected person may have transmitted the virus to two or three people, on average, early in the outbreak. But now, with Delta, that number could be five to nine.
This may be partly explained by Delta's ability to "replicate faster and lead to higher viral loads earlier in infection compared to other variants," according to the preprint study from Helix.
The CDC document also notes that Delta is "likely more severe" than what came before it.
Breakthrough infections may be contagious, but spread is driven by the unvaccinated
Prior to Delta, even if you developed a breakthrough infection, you'd likely have less virus in your airways. Thus, you might be less likely to infect someone else if you were vaccinated.
Experts say that vaccination still makes it less likely that you'll catch Covid-19 in the first place -- but for those who do, new data suggests that Delta causes similar viral loads in both vaccinated and unvaccinated people.
The data is based on an outbreak investigation in Massachusetts in which nearly three-quarters of infected people had been fully vaccinated. Of them, 8 in 10 developed symptoms, but only a few were hospitalized and none died.
"The idea that someone could still test positive and still develop enough virus in their nose and mouth to transmit is really what this data is showing," explains CNN's Chief Medical Correspondent Dr. Sanjay Gupta.
The finding that the Delta variant resulted in similar viral loads "was a pivotal discovery leading to CDC's updated mask recommendation," Walensky said Friday.
But viral load by itself may not be a definitive measure of contagiousness. For example, the immune systems of vaccinated people includes a number of moving parts that might impact how much they are able to spread it.
Walensky said that ongoing outbreak investigations will help uncover more about what happens when these breakthrough infections do occur.
"We are now continuing to follow those clusters to understand the impact of forward transmission of those vaccinated people," she said earlier this week. "But again, I want to reiterate, we believe the vast majority of transmission is occurring in unvaccinated people and through unvaccinated people."
Vaccination is the best tool against Delta
The effectiveness of Covid-19 vaccines is a spectrum: As you move from asymptomatic infection to hospitalization and death, effectiveness eventually reaches over 90%.
CDC documents estimate that vaccines reduce the risk of infection three-fold, and the risk of severe disease or death 10-fold or greater.
"Getting vaccinated continues to prevent severe illness, hospitalization and death -- even with Delta," Walensky said this week.
CNN's Gupta pointed out we've seen this trend reflected in the Covid-19 numbers on a national level.
"Last year when we would talk about a rise or surge in cases, almost predictably, you would see a certain percentage of hospitalizations that would follow thereafter, and a certain percentage of people who would die a few weeks after that," he said.
But the country has not yet seen the same magnitude of tandem hospitalizations and deaths. Experts say this metric is key when discussing if and when people might be advised to get a booster shot -- but as for now, the vaccines continue to hold up in this regard.
Masks are another necessary tool
More than 8 in 10 Americans live in a county with enough Covid-19 transmission for CDC to recommend people mask up indoors, even if they're vaccinated.
The move has already prompted a number of places, like businesses and school districts, to update their mask policies.
It's not just Delta's prevalence and contagiousness that motivated the CDC to update its guidance, but also lower uptake of Covid-19 vaccine than hoped.
"Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant," internal CDC documents say.
Similarly, research published Friday in Nature Scientific Reports maintains that vaccination alone won't stop the rise of new variants. The authors also warn of an "evolutionary arms race" where the virus may be allowed to spread and evolve among vaccinated people, potentially outsmarting our immunity in the future.
However, experts say we can bring the virus under control by layering strategies on top of vaccination -- masks especially, but also measures like distancing and testing.
"The measures we need to get this under control -- they're extreme. The measures you need are extreme," Walensky told CNN.
As more people get vaccinated, we'll see more breakthrough infections
Breakthrough infections were always expected. Simply put, no vaccine is 100% effective.
It also makes sense that, as more and more people get vaccinated, you'll see more breakthrough infections.
In April, mathematician and epidemiologist Adam Kucharski wrote on Twitter that these "patterns in highly vaccinated populations don't always do what you may assume."
As illustrated in CDC documents, a mathematical formula known as Bayes' theorem can give us some rough estimates:
In a population that's 60% fully vaccinated with an 80%-effective vaccine, you'd expect nearly a quarter of infections to be breakthroughs.
If you change either number to 70%, breakthroughs might approach nearly a third of infections.
This, of course, assumes everyone in a population is the same. You might see a disproportionate number of breakthrough infections, for example, in populations where vaccine effectiveness could be on the lower end -- like people who are immunocompromised or live in nursing homes.
But when we see percentages of vaccine effectiveness, that's talking about the population as a whole. It's not to say that any individual person is 70% or 80% or 90% protected.
For now, there's no surefire way to measure one's own immunity to the virus, despite a focus on antibodies. In fact, the country's leading health authorities advise against antibody testing in order to determine whether you're protected.
"The immune system is really complex and involves T cells as well as B cells," Dr. Monica Gandhi, an infectious disease specialist at University of California, San Francisco, told CNN in an email.
"Antibodies do not tell the whole story of immunity."
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