Virology and transmission
SARS-CoV-2 is a beta-coronavirus in the same subgenus as SARS-CoV-1 and some bat coronaviruses, Dr. Fauci explained. The viral genome is large, about 30,000 kilobases, and it has four structural proteins, most importantly the S or “spike” protein that allows the virus to attach to and fuse with cell membranes by binding to the ACE2 receptor on tissues in the upper and lower respiratory tract, gastrointestinal tract, cardiovascular system, and other organ systems.
The virus is transmitted mainly through exposure to respiratory droplets within 6 feet of an infected person, or sometimes through droplets or particles that remain in the air over time and various distances.
Contact with contaminated surfaces, once feared as a means of transmission, is now understood to be less common.
The virus has been detected in stool, blood, semen, and ocular secretions, although the role of transmission through these sources is still unknown.
“Some very interesting characteristics of this virus, really quite unique compared to other viruses, certainly other respiratory viruses, is [that] about a third to 40% of people who are infected never develop any symptoms,” Dr. Fauci said. “Importantly, and very problematic to what we do to contain it – particularly with regard to identification, isolation, and contract tracing – between 50% and 60% of the transmissions occur either from someone who will never develop symptoms, or someone in the presymptomatic phase of disease.”
The fundamentals of preventing acquisition and transmission are as familiar to most Americans now as the Pledge of Allegiance: universal mask wearing, physical distancing, avoiding crowds and congregate settings, preference for outdoor over indoor settings, and frequent hand washing, he noted.
Diagnostics
Tests for SARS-CoV-2 infection fall into three basic categories: molecular tests such as polymerase chain reaction (PCR) that are highly specific and highly sensitive for actual infections, antigen tests that detect the viral protein rather than the nucleic acids, and antibody tests to detect serum proteins made in response to viral infection.
Antigen testing is used largely for broader surveillance of groups of individuals to detect viral penetrance within that group, Dr. Fauci noted.
Clinical course
The clinical course of COVID-19 has some interesting characteristics but is not substantially different from a flu-like syndrome, Dr. Fauci said.
Symptoms and signs common to both types of infections include fever, cough, fatigue, anorexia, dyspnea, and myalgias, but the loss of smell and/or taste preceding the onset of respiratory symptoms is a unique feature of COVID-19.
Dr. Fauci cited data on more than 44,000 individuals with confirmed COVID-19 in China that showed that a large majority (81%) of cases were mild or moderate in nature, but 14% of patients experienced severe disease, and 5% were critically ill. The case-fatality rate in this study was 2.3%.
People at increased risk for severe disease include older adults and those of any age with certain comorbidities.
Manifestations of severe COVID-19 infections in adults can include neurological disorders, hyperinflammation, acute respiratory distress syndrome, cardiac dysfunction, hypercoagulability, and acute kidney injury.
In children, COVID-19 has been associated with a multisystem inflammatory syndrome (MIS-C) similar to Kawasaki disease.
In a substantial number of cases, the effects of COVID-19 can linger for 6 months or longer, Dr. Fauci said, pointing to a study from the University of Washington in Seattle.
Investigators there found that approximately 30% of patients enrolled at their center reported persistent symptoms for as long as 9 months after the initial illness, with fatigue as the most commonly reported symptom. One-third of outpatients with mild disease also reported persistent symptoms.