Covid cases in the US have doubled in the past two weeks and scientists are now rushing to understand the delta variant, which seems to explain the vast majority of new infections. Worryingly, delta is more contagious than other variants and also causes some symptomatic “breakthrough” cases in vaccinated people.
While vaccines still predominately prevent serious illness and death, the delta variant has changed the way we think about the spread of the coronavirus. Here are some answers to a few key questions about what it all means.
1. What makes the delta variant more contagious?
According to estimates from the CDC, the delta variant is nearly twice as contagious as previous versions of the virus. Researchers are still trying to understand the mutations that make this happen, but preliminary studies suggest that changes in the spike protein make it more efficient at both grabbing receptors and getting into your cells.
The delta variant also seems to lead to a higher viral load than other variants. Viral load is a measure of how much virus is in your nose and throat. a study found that at the beginning of their infection, people with the delta variant had a viral load 1,000 times greater than that of people infected with the original version of the virus. People with the delta variant also reached their peak viral load faster, according to this study, which has not yet gone through peer review.
2. How do scientists actually measure how contagious the delta variant is?
Viral load helps us understand how contagious a virus is. Coronavirus infections spread through aerosols and droplets when an infected person coughs, sneezes or simply breathes – so the more virus particles in a person’s airways, the more likely that person is to infect someone else.
To measure viral load, researchers use a lab method called polymerase chain reaction, or PCR. They take an infected person’s nose and extract any viral RNA that’s on the smear. Then they run the reaction, which seeks out virus genetic material and copies it over and over until there are enough copies for the lab equipment to detect.
We usually focus on the endgame of PCR – or finding a test material from a virus, which gives a positive result. But researchers can also look at how long it took the machine to return that positive result — how many copies it took to get the viral material to a detectable level. The fewer copies or cycles it took to detect a virus, the more viral material there was to begin with.
This number of cycles, called the cycle threshold, or Ct, is the number that raised eyebrows at the Centers for Disease Control and Prevention. In a cluster of business in Provincetown, Massachusetts, approximately 74% of affected state residents were vaccinated. People who got infections tended to have similar Ct levels whether they were vaccinated or not. The CDC thought this could be an indication that: vaccinated people can transmit the virus, perhaps as easily as unvaccinated people.
3. Can I still get sick from covid, even if I have been vaccinated?
Yes, it is possible, although your infection will likely be much less severe than an unvaccinated person’s.
The vast majority of infections still occur in unvaccinated people, says Liz Rogawski McQuadec, an infectious disease researcher at the University of Virginia. According to reports from the Kaiser Family FoundationUS states that monitor the vaccination status of cases find that between 94% and 99.9% of cases are unvaccinated people. And of everyone who has been vaccinated, between 0.01% and 0.54% have experienced a breakthrough case.
Some studies have found that vaccine efficacy is slightly lower against the delta variant, especially if you have only received one dose of an mRNA vaccine. But so far it seems vaccines still largely work, especially in preventing many cases of serious illness, says Rogawski McQuade.
Vaccines may eventually need some extra help against the delta variant – some companies do insist on booster shots. But experts say there’s no evidence yet that boosters are needed, and the WHO maintains that initial vaccines for the rest of the world should take precedence over booster injections for people in rich countries.
4. What about the transfer? Can vaccinated people spread the delta variant?
It seems so, but the research is still in the early stages.
While Ct levels can be used as a proxy for viral load, there are a few problems with trying to assume too much based on that number, especially when it comes to vaccinated people, according to Monica Gandhi, an infectious disease researcher at the University of California, San Francisco.
First of all, PCR picks up all kinds of genetic material, even from dead viruses. If your vaccinated immune system has started fighting the infection, “you may have a lot of viral particles in your nose, but they may not be working,” Gandhi says. To really know how contagious someone is, you have to take those viruses and see if they’re still alive and can infect people. The CDC has noted that this data is still excellent, Gandhi says.