“Super Carriers” – 2% of People Carry 90% of COVID-19 Virus

A lab scientist scans barcodes on saliva samples collected from members of the CU Boulder community. Credit: Glenn Asakawa / CU Boulder

A few ‘supercarriers’ with off-the-charts viral loads are likely responsible for the majority of COVID-19 transmissions, while about half of infected people are not contagious at all at the time of diagnosis, new CU suggests Boulder analysis of more than 72,000 test samples.

A second, related study gives even more credence to the idea that viral load, or the amount of virus particles a person carries, causes contamination. The survey found that only one in five college students who tested positive while living in a residence hall infected their roommate. And their viral load was nearly seven times that of those who did not spread the virus.

“The conclusion of these studies is that most people with COVID don’t make other people sick, but a few people make many people sick,” said Sara Sawyer, professor of molecular, cellular and developmental biology and senior author of the first study. If you don’t have a viral supercarrier around at dinner, you might be fine. But if you do, you’re out of luck. It’s a game of roulette so you have to be careful. “

For the studies – one of the largest to date to examine trends in asymptomatic people – researchers analyzed saliva samples collected from students, and some faculty and staff, at the CU Boulder campus between Aug. 17 and Nov. 25.

Asymptomatic students in residence halls are required to test weekly, using a free, highly sensitive saliva-based screening test called RT-qPCR (reverse transcription polymerase chain reaction), which detects and quantifies genetic material from the virus that causes COVID-19.

A member of the CU Boulder community hands over a saliva sample for COVID-19 surveillance testing. Credit: CU Boulder

In the fall, of the 72,500 samples taken from asymptomatic humans, 1,405 cases of COVID-19 were identified.

“What’s so special about these samples is that they all come from infected people with no symptoms – a snapshot of all these seemingly healthy people who you assume are safe to be around,” Sawyer said.

Super carriers = super spread

Sawyer and her team quantified how many viral particles or virions each of those samples contained, drew it up, and compared it with samples from hospitalized patients. Some surprising patterns emerged. First, the distribution of viral loads in the asymptomatic sample was indistinguishable from that of highly symptomatic patients.

“This means that the symptoms tell you little about what’s going on in a person’s body,” Sawyer said. “Some of these asymptomatic people have viral loads as high as someone intubated with COVID in a hospital bed.”

Only 2% of all COVID positive individuals at UC Boulder carried 90% of the circulating virus. A student with the highest load contributed 5%.

Meanwhile, about half of those who tested positive had such a low viral load (less than 106 virions per milliliter) that they probably were no longer carrying live virus – instead, they may have just shed viral fragments from tissue under repair. So they probably weren’t contagious.

“This is another example of why you don’t necessarily need super sensitive tests that can take longer,” said study co-author Roy Parker, director of the BioFrontiers Institute and Howard Hughes Medical Institute investigator. “Even a faster but less sensitive test will catch all infectious humans.”

Dorm, with precautions, still safe

For a second study, researchers used the same samples to examine how often one roommate infected another.

In total, 1,058 students living in the dormitories tested positive, representing 16.5% of the population.

Students in single rooms were about half less likely to be infected. But this was not because the virus was spreading between roommates. (Previous research has shown that students living alone generally have fewer social contacts).

In fact, only 20% of infected students – those with significantly higher viral loads – have transmitted the virus to their roommates.

Notably, CU Boulder – while students could have roommates in the fall – required students diagnosed with COVID-19 to move to a dedicated isolation dormitory for 10 days. But it may take a while for a student marked by screening to get a follow-up diagnosis, be notified, and leave.

“You might think that students who lived with another student for longer before isolating them were more likely to transmit the virus to their roommates, but we saw no impact,” said lead author Kristen Bjorkman, COVID’s scientific director for CU Boulder.

This doesn’t mean that isolation doesn’t affect the spread of the virus at all, she said, but it does deliver a glimmer of optimistic news for people who want to live with other people but are concerned about safety.

“This is important for us and other universities to know, as it tells us that we can continue to offer on-campus housing and matching roommates,” she said.

The findings may also provide relief for those who have received a positive COVID-19 test and fear of people they live with.

“People feel very guilty about this, but our study suggests that getting a positive test isn’t a guarantee that you’ll infect your loved ones or roommates,” Bjorkman said.

Neither article has been published in a peer-reviewed journal to date.

Collectively, the research shows that in some cases it may be wise to reach out to people with high viral loads quickly – and encourage them to isolate themselves quickly.

“This could go a long way in preventing major outbreaks,” said Bjorkman.