- The highly infectious Delta variant may cause higher rates of breakthrough infections, but fully vaccinated people are still well protected against severe illness.
- After a sharp drop in coronavirus cases throughout the first half of the year, cases began to spike again in July as Delta took over. This variant now accounts for the vast majority of new infections.
- The Centers for Disease Control and Prevention warned that Delta is “likely more severe” than earlier versions of the coronavirus.
- An outbreak in Provincetown, Massachusetts, that included vaccinated people helped reveal how infectious the Delta variant could be.
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The Delta variant of the coronavirus has quickly changed the direction of the COVID-19 pandemic in the United States.
After a sharp drop in coronavirus cases throughout the first half of the year, cases began to spike again in July as Delta took over. This variant now accounts for the vast majority of new infections.
This is occurring even with almost half of people in the United States being fully vaccinated — although that means over half are unvaccinated, including children under 12, who aren’t yet eligible for the COVID-19 vaccine.
Still, the variant has challenged the country’s pandemic response in ways that were unexpected a few months ago.
The Centers for Disease Control and Prevention (CDC) warned that Delta is “likely more severe” than earlier versions of the virus, according to an internal report made public on July 30. Researchers caution that more work is needed to sort this out.
But the Delta variant has already proven itself to be highly transmissible — on par with chickenpox, the CDC said in its report — with cases rising sharply throughout the United States, especially in areas with low vaccination rates.
A recent study published in July as a preprint also found that people with a coronavirus infection from Delta carried 1,000 times more virus in their bodies than people with an infection from the original coronavirus variant.
As a result of the variant’s high transmissibility, states like Arkansas, Florida, Missouri, and Florida are seeing sharp increases in cases, with rising hospitalizations not far behind.
The variant also appears to be causing higher rates of breakthrough infections — or infections in people who are fully vaccinated — than earlier versions of the virus, with concerning signs that some vaccinated people may be able to easily transmit the virus.
An outbreak in Massachusetts made headlines after many vaccinated people were affected, although very few were hospitalized for the illness and none died.
In spite of these challenges, the COVID-19 vaccines approved in the United States still offer strong protection against severe illness, hospitalization, and death caused by Delta and other variants.
But ongoing surges of the Delta variant put unvaccinated and immunocompromised people at risk, threaten to overwhelm hospital systems, and increase the chance of another variant emerging.
Here are some key things to know about the Delta variant’s transmissibility and how it’s affecting the COVID-19 vaccines.
Some groups, such as immunocompromised people and older adults, are at higher risk of breakthrough infections because they may have a lower immune response after vaccination.
The risk of the virus spreading to vaccinated people also increases when many people gather close together for extended periods — especially when unvaccinated people are present.
The CDC reported on one outbreak that occurred in Provincetown, Massachusetts, following several large public events.
Almost 470 COVID-19 cases occurred among state residents who traveled to Provincetown for those events. Of those, 74 percent occurred in fully vaccinated people.
Testing also showed that 90 percent of the cases were caused by the Delta variant. The results were published July 30 in the CDC’s
Based on this data, “it is very clear that the Delta variant is transmitting despite vaccination,” said Dr. Edward Jones-Lopez, an infectious diseases specialist at Keck Medicine of USC. “That is obviously very concerning.”
Although the Provincetown outbreak raises concerns about the risk of breakthrough cases caused by Delta, many fully vaccinated people attended those public events without contracting an infection.
Health officials have also been sounding the alarm about the risk of Delta variant-driven outbreaks in long-term care facilities.
Although the majority of residents are fully vaccinated, in some places the rates are lower among employees, which can lead to breakthrough cases.
No vaccine is 100 percent effective, so some fully vaccinated people will contract an infection.
The Delta variant appears to be causing breakthrough infections at a rate higher than earlier variants of the virus, according to the latest CDC report.
The agency also cited several studies of the Pfizer-BioNTech COVID-19 vaccine that showed the vaccine has a lower effectiveness against infection caused by the Delta variant compared with the Alpha variant.
Even as the Delta variant expanded its hold on the country over the past few months, breakthrough infections since January have been relatively rare.
NBC News estimates that during that time, there have been 125,682 breakthrough cases in 38 states out of around 164 million people fully vaccinated.
This comes to about 0.08 percent of fully vaccinated people who had tested positive.
Or to put it another way, almost 100 percent of vaccinated people have not had a breakthrough infection.
Most of these breakthrough cases have been mild. The
There are likely additional cases so mild that they went unnoticed and unreported.
In the Provincetown outbreak, some vaccinated people who contracted an infection had similar levels of viral RNA in their upper airways — aka noses and throats — as unvaccinated people who contracted an infection, according to CDC researchers.
However, the RT-PCR tests used in this study only show how much viral RNA is in the sample, not whether vaccinated and unvaccinated people with an infection were equally contagious.
“It’s really important to note that RT-PCR measures viral RNA, NOT infectious virus,” Angela Rasmussen, PhD, a virologist at the University of Saskatchewan, wrote on Twitter.
However, the CDC and other health experts think vaccinated people play a small role in transmission.
“Vaccinated people can transmit Delta if infected. However, the majority of transmission is still by UNVACCINATED — that’s where the focus should be,” Dr. Leana Wen, an emergency medicine physician at George Washington University, wrote on Twitter.
The reason we don’t yet know about the impact of the vaccines on transmission is because the focus of the early vaccine trials was on preventing symptomatic infection and severe illness — which are both easier to study.
“The study design needed to answer the [transmissibility] question is very complicated and very expensive,” Jones-Lopez said. “But now with the emergence of the Delta variant, the question has become even more relevant.”
Some studies suggest that the vaccines have some impact on transmission.
But this study was done when other variants were dominant in the country.
Other research groups have ongoing transmission studies.
One of these, organized by the COVID-19 Prevention Network, is being conducted at more than 20 universities around the country. This study should have results within the next few months.
Although more research is needed before we know how much the COVID-19 vaccines block transmission, Rasmussen wrote on Twitter that it’s the “right call” to assume that vaccinated people can transmit the virus, and to “take precautions” such as wearing masks indoors.
But “it’s also important to note the main risk here is transmission to unvaccinated people,” she added.
Vaccinated people are still well protected against severe illness.
“Data from the CDC shows that over 99 percent of people who are in the hospital or who have died from COVID in the last several months have been unvaccinated,” Jones-Lopez said.
Amid an increase in people with COVID-19 dying, a Missouri hospital recently had to expand its morgue capacity.
This is happening in a country with enough vaccine doses to inoculate its entire eligible population.
Fully vaccinated people are also 25 times less likely to be hospitalized or die due to COVID-19 than unvaccinated people, according to the recent CDC report.
While some fully vaccinated people will get breakthrough infections, if they hadn’t been vaccinated, their illness might have been much more severe.
In addition, as hospitals and ICUs in some counties fill up with COVID-19 patients, other medical care could be impacted, including emergency care for car accident victims and other non-COVID-19 care.
The best thing that people can do to protect themselves from the Delta variant is to get vaccinated.
Rasmussen said nonpharmaceutical interventions such as masking can also add additional barriers to the virus. These complement the protection offered by vaccination.
“If enough people are vaccinated AND taking precautions to reduce exposure, even Delta will hit too many dead ends to continue spreading in the population,” she wrote on Twitter.
“That’s what we need to aim for because we’ll never achieve 100 percent vaccination, even after kids [under 12 years] are eligible,” she wrote.
The new findings about the high transmissibility of the Delta variant led the CDC last week to advise that all people — vaccinated and unvaccinated — wear a mask in indoor public spaces in areas with high rates of coronavirus transmission.
Slowing the spread of the virus through vaccination, masking, and other measures not only has benefits for the individual, but also for the community, both locally and globally.
Not every American who is unvaccinated is opposed to the COVID-19 vaccines.
People who are immunocompromised need others to get vaccinated to help shield them from the virus.
And children under 12 are not yet eligible for vaccination. Although they have a lower risk of severe illness than older adults, many pediatricians don’t see COVID-19 as a benign illness for kids.
Controlling transmission will also ensure that children and teens are able to safely return to school in the fall. Outbreaks during the first week of school already forced one district in Mississippi to return to virtual learning.
There also remain large racial and other disparities in the vaccination efforts in the United States, including difficulty getting time off from work or traveling to a vaccination site.
Reducing transmission can even have a direct impact on the coronavirus itself.
“Vaccinating as many people as possible will be the best way to prevent the emergence of new variants,” Jones-Lopez said, “ones that are potentially going to be even worse than the Delta variant.”