NEWS | August 5, 2020

COVID-19 insights from the expert leading UC Davis Health’s coronavirus fight

40 years of treating infectious disease gives Stuart Cohen full perspective on coronavirus


Near the end of a virtual COVID-19-themed town hall for the UC Davis Health community in April, David Lubarsky, CEO of the health system, brought out a special gift for a unique man.

Stuart Cohen leads UC Davis Health’s fight against COVID-19. Stuart Cohen leads UC Davis Health’s fight against COVID-19.

The gift: A cart stuffed with 100 Diet Cokes. The recipient: Stuart Cohen, chief of the Division of Infectious Diseases and director of hospital epidemiology and infection control. Cohen has been leading UC Davis Health’s effort to protect the hospital and the community. Lubarsky said he knew Cohen could use a good supply of his favorite picker-upper.

“The man works endlessly,” Lubarsky said, “(He is) tirelessly working on behalf of the patients, our employees and the safety of our community.”

Cohen wasn’t there to receive his gift. He was working.

Caffeine aside, Cohen’s leadership for UC Davis and the region in the fight against SARS-CoV-2 and his nearly 40 years at UC Davis Health put him in position to explain both what we know about COVID-19 and how it compares with other dangerous diseases. Through his career, Cohen has worked to find answers to some of medicine’s most vexing problems, including HIV/AIDS, drug-resistant microorganisms, H1N1 and West Nile virus.

The battle against fear

“I finished my training during the start of the AIDS pandemic. There was similar fear and anxiety as when this started,” Cohen said. “It took almost two years to determine the cause of AIDS, only then did that start to calm some fears in the hospitals.

“With COVID-19, not everybody dies, but it’s much harder to protect yourself,” he said. “It’s so much more transmissible. But we’re learning so much faster, and we have much more experience managing the fear.”

On the other hand, he said, now health experts find themselves in unexpected disputes over basic science as they try to manage that fear and the coronavirus.

“When we were transmitting messages about AIDS, there wasn’t social media. There weren’t all those places where people get information that isn’t factual,” Cohen said. “We’ve always had to fight against misinformation, but social media magnifies that.”

Making people aware that the research is still evolving because the novel coronavirus is so new has become a public health challenge, he said.

“We gain knowledge every day about COVID-19,” Cohen said. “Bringing the public along is important, but it’s much more difficult than it should be.”

The scale of the pandemic

“We haven’t seen anything this transmissible in our lifetime outside of measles and chicken pox,” he said. “I hear people talk about mortality rates being low, but 160,000 people have died in the U.S. because it’s so easy to spread. The percentage doesn’t mean much when the totals are so high, or if you’re one of those people who dies.”

UC Davis Health CEO David Lubarsky gave Stuart Cohen his favorite picker-upper to help in his tireless work against the coronavirus.

Infectious disease scientists use the term “R naught” to explain the average number of people that one sick person will infect. For COVID-19, that number is 2.5. It means each person with the coronavirus will, on average, infect 2.5. people, and each of those people will infect another 2.5 people. Very quickly, the numbers become enormous, as we have seen. More than 500,000 Californians and nearly 5 million Americans have been infected with COVID-19.

“For influenza, R naught is under 1, maybe close to one-half a case from every sick person,” Cohen said. “Measles is 18 and chicken pox is 12, but those are the only ones higher than COVID-19.”

Forget about waves

“Any talk about waves has become less relevant because we never got things under control,” Cohen said. “Most coronaviruses tend to slow in summer, but we don’t know why some virus are less seasonal. This is one of them.”

People’s movements play a role. In summer, people are outside more and less packed together. But Northern California, like much of the country, got worse as the weather warmed because many people actually spent more time together in June and July.

“As good as we were doing in Sacramento, we lost control,” he said. “If we had opened a little more slowly, it would have been different.”

Meaning, we’re still being hit by one large, continuing wave of COVID-19. Yet, it’s still likely to be worse in fall, when people will likely be closer together indoors.

“If people don’t socially distance and wear masks, we are very likely going to see a big kick in the fall,” Cohen said. “And we’ll be dealing with flu season at the same time.”

When are you most infectious?

“The incubation period varies because it depends on how much virus you were infected with,” Cohen said. “You’re probably most infectious a day or two before you actually have symptoms, then for a couple days after. Mostly it’s a four-day window around when you start getting sick.”

That’s why he advises assuming everyone might be spreading COVID-19. Some of the infectious window includes a few days when people don’t appear to be sick. Also, two people could be exposed at the same event, but one person took in more virus and will get sick in a few days while another may start to show symptoms two weeks later.

“It can take the virus a little time to replicate enough in your body so that you develop symptoms,” Cohen said. “If you get sick on day 14, you’re most infectious on days 12 to 16. If your friend gets sick on day seven, they’re most infectious days 5 to 9.”

How long do people remain infectious?

The World Health Organization’s latest guidelines for releasing COVID-19 patients from isolation say someone is not likely to transmit the virus 10 days after they started having symptoms, plus add another three days without symptoms.

But Cohen pointed to a new study to be published in the November edition of the Emerging Infectious Disease journal from the Centers for Disease Control and Prevention. It says that for mild cases, a person who is done showing symptoms is likely not infectious nine days from the start of their symptoms. The study says those people are very unlikely to transmit the virus, even if they still test positive, because they will have such low levels of the coronavirus and they are producing virus neutralizing antibodies.

However, the study said those people should still be kept away from vulnerable populations. It also said people with more serious cases of COVID-19, or with weakened immune systems, will remain infectious longer.

Why some people get much sicker than others

Some people get seriously sick because of weakened immune systems. Others face worse cases because of the amount of virus they took in. But some of it is unpredictable.

“It’s biology,” Cohen said. “Everybody is different, and their immune response to disease is different. Every infection has the same variations. If you get the flu, you might have a serious case and someone else is milder.”

For the sickest people, it’s a mixture of damage caused by the virus and by their own bodies because their immune systems overreacted.

“Scientists talk about cytokine storms,” he said. “That’s when your immune system overshoots and your body attacks its own cells and tissues, not just the virus.”

What is the risk to younger adults?

More than 60% of the COVID-19 cases in Sacramento County are among people age 18-49, and the largest single group is age 20-29. This trend has been a concern for infectious disease experts like Cohen — partly because any lack of caution among younger people will spread the coronavirus to more vulnerable groups, and partly because many don’t realize their own vulnerability.

“As a group, young adults are one of the reasons COVID-19 is still out of control. What we want them to understand is that there are some young people who do very badly with the disease.”

“As a group, they are one of the reasons COVID-19 is still out of control,” Cohen said. “What we want them to understand is that there are some young people who do very badly with the disease.”

Although younger adults with strong immune systems tend to have milder cases, the number of serious cases among them has grown because so many people in that age group have been infected.

“That means many more of them are in danger,” he said. “So are the people they come in contact with.”

The problem with immunity

“The duration of immunity is difficult to determine,” Cohen said. “It may relate to how sick you were to start with. People who had milder illnesses may have a shorter duration of immunity. This will take a while to work out.”

The reason: There is no short cut. Immunologists have to watch people, their antibody levels and the coronavirus over actual time to understand how long immunity lasts.

“We need that time,” he said. “It will take a year or two to know.”

This may not be the last pandemic

In a way, Cohen and his team saw this coming. They did not predict the actual coronavirus or dates, but they’ve been watching infectious diseases move from animals to humans as the world’s environment and habitats change. Increased contact between animals and people means increased disease.

“If you look at what’s happened over the last 20 to 30 years, there have been a number of new infections,” Cohen said. “We’ve had SARS, bird flus, Ebola, Lyme disease and others. For that matter, HIV was an animal virus transferred to humans.”

And more are likely, he said.

“We encroach on all the places animals, and the viruses, live,” Cohen said. “We know many viruses and other pathogens jump to human. It all depends on how well the virus adapts to humans as a host. That determines whether it develops into a pandemic.”

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