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The coronavirus pandemic defined our lives in 2020. Here’s what we know and don’t.

CasPhotography/iStockBy MEREDITH DELISO, ABC News

(NEW YORK) — On Jan. 9, the World Health Organization announced that a new coronavirus could be behind a mysterious disease that had sickened 59 patients in Wuhan, China.

Almost a year later, what is now known as COVID-19 has infected more than 80 million people globally. More than 1.7 million people have died, including more than 333,000 in the United States, according to Johns Hopkins University, and our lives have been fundamentally altered.

Infectious disease experts, scientists and medical professionals have worked tirelessly over the past year to determine how this previously unknown virus is transmitted, how to prevent it from spreading and how to treat it.

“Everything is new for this, that’s for sure,” Justin Lessler, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health, told ABC News.

As knowledge of COVID-19 continues to evolve, here’s a look at some key findings so far.

How COVID-19 is spread

Looking to other coronaviruses — a large family of viruses that includes the common cold — has helped shape our initial understanding of SARS-CoV-2, as the novel virus is officially known. One such assumption that’s been borne out, Lessler said, is that COVID-19 is primarily transmitted through large droplets and aerosolized particles.

COVID-19 transmission, we have also learned, is “overdispersed,” Lessler said, meaning that a small number of individuals is responsible for a large number of infections — something scientists did not expect.

“What that means popularly is that they’re ‘super-spreading’ events,” he said. “Most people tend to only infect a small number of individuals, or no one, whereas some people end up infecting a lot of people.”

Much of that is due to the situation. Indoor gatherings where a lot of time is spent unmasked, with potentially poor air circulation, “are incidences we know to be very conducive to spread,” Lessler said.

The virus can be transmitted on surfaces and through aerosolized fecal matter, though those are “much less significant,” Lessler said.

To that end, mask-wearing — a relatively novel concept in the U.S. and Western world up until this point — is important to limit the spread of infection, Dr. Jessica Justman, associate professor of medicine in epidemiology at Columbia Mailman School of Public Health and senior technical director at ICAP at Columbia University, told ABC News.

“What we would call in the world of infection control ‘contact precautions’ seems less important,” she said, referring to surface transmission. “I’m not saying to ignore those recommendations — it’s still important to wash your hands and be careful about what you touch — but I think that mask-wearing is much more important than the contact precautions.”

A year into COVID-19’s emergence, it’s also become apparent that there is likely some seasonal component to transmission, Lessler said.

“It does seem like it’s following that typical respiratory virus pattern of being more transmissible in the fall,” he said. “The evidence suggests it’s not merely an effect of people becoming less careful in their control. It’s also an effect of the virus transmitting more efficiently.”

Symptoms of COVID-19

One of the most important things learned about COVID-19 symptoms is their potential absence in those who are infected. It’s still unclear how often people are truly asymptomatic, though it could be as many as one in three, Lessler said. Justman puts it as high as 45% of cases.

“I think we have learned that asymptomatic cases are far more common than we had expected at the beginning,” Justman said.

Those who do ultimately develop symptoms also may spread when they are asymptomatic, Lessler said — something that has made COVID-19 “so successful” at moving through the population.

“People who develop symptoms, maybe a third to half of … people they infect, they infect before they themselves ever have symptoms significant enough to feel like they’re sick,” he said.

Mild to severe symptoms may appear anywhere from two to 14 days after exposure to the virus, according to the Centers for Disease Control and Prevention. At first, the CDC had only listed fever, cough and shortness of breath as possible symptoms, though that list has since expanded to nearly a dozen, including fatigue, body aches, headache nausea and diarrhea.

A “variety of unusual symptoms,” such as loss of taste or smell, has also been discovered, Justman said. Patterns and trends of skin conditions, such as “COVID toes” (frostbite-like areas of typically red or purple discoloration can appear on the feet) and rashes, have also been reported.

What COVID-19 has taught us

As much as we’re learning about COVID-19, the pandemic has brought home some lessons as well.

One is the “critical role” of clear communication by public health officials and governmental authorities, Justman said, particularly as it pertains to support of measures such as social distancing and mask-wearing — basic mitigation measures with which the U.S. was not acquainted.

“At the beginning of this pandemic, a lot of experts were saying, ‘No, don’t wear a mask,’ and that’s certainly led to a lot of confusion,” Justman said. Public health officials were initially concerned about a run on medical-grade respirators and other masks that would leave front-line workers without adequate protection.

“As the mask supply improved, and as our knowledge expanded and we had a much greater understanding of the importance of in particular respiratory droplets, then the message changed,” Justman said.

COVID-19 also showed us how low the surge capacity is in our healthcare system, Lessler said.

“We’ve never gotten things as bad as they could have been, but really, a lot of hospital systems, particularly early on, were quickly overwhelmed,” he said. In New York, the original epicenter of the pandemic in the U.S., field hospitals were established, including a Navy hospital ship.

The development of a COVID-19 vaccine has also shown the effectiveness of mRNA, or messenger RNA, technology. Both vaccines from Moderna and Pfizer, which received emergency authorization, utilize mRNA — meaning they teach cells to make a protein that prompts an immune response — and both have proven to be highly effective in trials.

“We didn’t know that before,” Justman said. “It was not a vaccine strategy that had been used before in humans. So it’s been great to see how successful that is.”

That a highly effective vaccine could be made so quickly is another lesson during the pandemic, Lessler said. The previous record was four years.

“Many people would have told you back in September even, that though things were looking good from the immunogenicity studies, there was no guarantee we would be able to have an effective vaccine against the coronavirus, it just had not been done before,” he said. “So that is very good news.”

What we still don’t know

As we learn more about COVID-19, there’s still much more that warrants continued study. One area is around the transmissibility of different strains of the virus — particularly amid reports of two new variants detected in the UK that seem to spread more easily.

“In trying to work out exactly how different mutations affected transmissibility, I suspect we’ll be working on that for decades to come,” Lessler said.

There’s also more to learn about how long immunity lasts, whether from a vaccine or natural infection, he said.

Natural immunity, gained from having an infection, “varies from person to person,” according to the CDC. “Some early evidence suggests natural immunity may not last very long,” it said. More data is needed on vaccine immunity, it noted.

The effectiveness of various COVID-19 treatments, such as monoclonal antibodies and convalescent plasma, also warrants more study, Justman said.

Though over months of treating patients with COVID-19, hospitals have learned some effective treatment techniques, such as proning — turning those with respiratory ailments onto their stomachs. At the beginning of the pandemic, early ventilator use was considered a best practice, and states scrambled to procure a limited supply of the machines. Though only about a third of COVID-19 patients on ventilators survived, according to the Intensive Care National Audit and Research Centre, and doctors have since gained a better understanding of when to place extremely ill patients on one. The use of the steroid dexamethasone has also shown positive results in seriously ill patients.

One of the more perplexing aspects of the virus concerns “long-haulers” — those who experience lasting symptoms even after the virus has left the body.

“It’s really not fully understood,” Justman said. “It may affect up to 10% of people who had symptomatic COVID, but this is still very much understudied.”

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