People with COVID-19 infection, but without symptoms, probably cause much of the transmission to others. Epidemiological investigations in Chicago (Ghinai, Woods et al.), China (Bai et al.; Bi et al.; Columbia University; Du et al.; Li C, Wang, et al.; Li R, Pei, et al.; Pan X, Chen, et al.; Wang Y, Tian, et al.), Germany (Rothe et al.), Japan (Nishiura, Kobayashi et al.), and Singapore (Wei WE, Li, et al.; Yong SEF et al.) of patients’ contacts with other people found many probably received infection from asymptomatic carriers or presymptomatic patients. Many infected people with few or no symptoms shed as many viruses as symptomatic patients (He et al.; Zou et al.).
Several studies found large percentages of people had asymptomatic infections, varying with their ages and transmission situations (Oran, Topol). For an example, in the general population of a high transmission area, among pregnant women admitted to a New York City hospital for birth delivery, 14% had SARS-CoV-2 (Sutton et al.). After several residents of a long-term care skilled nursing facility in Washington state had coronavirus COVID-19, scientists tested 93% of residents, and found that about half of those infected did not show symptoms (Kimball et al.). When large numbers of cruise ship passengers and crews were tested for viral RNA, 26% – 51% of those infected were asymptomatic. When they tested the same individuals repeatedly, 17.9% of those infected never showed symptoms (Mizumoto et al.). So, much of the transmission on the cruise ships was probably from asymptomatic people (Moriarty et al.). Many children with few or no symptoms shed large numbers of coronaviruses, and so could be transmitting it to adults, who could later develop severe infections (Kelvin and Halperin; Qiu et al.; Van Beusekom, “COVID-19”; Xu et al.).
COVID-19 can spread more easily than many other infectious respiratory diseases, including in the incubation period and asymptomatic infections (Gandhi et al.). Symptoms usually start about 5 days after exposure, ranging from 2 -14 days. This virus quickly multiplies in the person, and produces millions of viruses in mucous and saliva in a few days. After symptoms stop, the virus is still shedding from that person for up to 49 days. Patients start to develop immune antibodies in a few days after exposure, but the viruses continue multiplying and shedding, and gradually decrease later (Lauer et al.; National Academies of Sciences, “SARS-CoV-2 Viral shedding”; Tan et al.; Van Beusekom, “Study”; Wölfel et al.; World Health Organization “Immunity…”).
Altogether, people with COVID-19 symptoms can probably transmit to others about two days before symptoms begin, and then for about another 7-10 days. After symptoms start, SARS-Cov-2 replication probably continues for about a week in most patients, then usually decreases so viruses could not be cultured from most patients after two weeks. But weeks later some patients might still have non-infectious RNA found by PCR (National Centre for Infectious Diseases). Consequently, people who have been exposed to an infected person, but do not show symptoms, should prevent transmitting to others. They should remain quarantined, avoid contaminating surfaces or coughing in rooms used by others, for 14 days.
People who live with COVID-19 patients should protect themselves for weeks afterwards. When scientists followed up both ordinary COVID-19 patients (Bai et al.; Hu et al.; Xing et al.) and medical staff patients (Lan et al.), they found that most of those recovered had no SARS-CoV-2 RNA, then later started having viral RNA again. Their families living with them used infection prevention methods, but some of their family members were infected.
After people have COVID-19, their long-term reactions vary. Many convalescent patients developed SARS-CoV-2 neutralizing antibodies, which decreased months later (Klar; Lei et al.; Long et al.; Seow et al.; Wajnberg et al.). Memory T cells and B cells, which are kinds of white cell lymphocytes, develop and provide longer-lasting immunity (Calhoun et al.; Grifoni et al.; Mandavilli; Ni et al.; Robbiani et al.; Sekine et al.; Thieme et al.). Consequently, patients with no detectable antibodies might still be immune. Those with antibodies might still develop symptoms again later.
Some had COVID-19 symptoms for months, most testing positive for viral RNA, and some not. It was unclear if some transmitted to others or not (Garner; Mandavilli; Yong “COVID-19 can last..”). A follow-up of those who had symptoms, tested positive, recovered, and tested positive again found that none had transmitted to any contacts (Korea Centers for Disease Control).
Doing controlled tests on humans to discover how many developed immunity would be difficult and not-very-ethical, so scientists have been experimenting on animals who get COVID-19 similarly to humans, including hamsters (Chan, Zhang et al; Imai et al.; Sia et al.) and rhesus macaques (Chandrashekar et al; Deng, Bao et al.; Yu, Qi et al.) They found that some hamsters (Imai et al.) and rhesus macaques (Chandrashekar et al; Deng, Bao et al.) infected with COVID-19 developed antibodies and were protected against getting infected again.
These patterns of symptoms, RNA, and viruses could allow many asymptomatic, presymptomatic, and post-symptomatic people to continue transmitting to many others. This could permit the pandemic to continue for many months, and resurge in communities that relax social distancing, testing, and treatment.
Exhaling indoors for hours
Contact tracing studies of who infected whom found many episodes in which people interacted indoors for hours, and few with transmission outdoors (Bromage; Frieden, Lee; Nishiura, Oshitani, et al.; Qian et al.). The previous paragraph described patients transmitting to their families at home. In other episodes, people were breathing, coughing, sneezing, yelling, and touching each other (Asadi et al.; Atkinson et al.; Tang JW, Li et al.) in restaurants (Lu J, Gu et al.), a telephone call center (Park et al.), choir rehearsal (Hamner et al.; Read), indoor sports (Dawson), and family gatherings (Bromage; Ghinai, Woods et al.). Close, prolonged contact indoors with presymptomatic and symptomatic cases caused several cluster outbreaks (Cevik; Cevik, Bamford, Ho). The relationships most likely to transmit were: friends, family members, taking the same transportation, living with each other, being within 6 feet of each other, and eating together, more likely than brief contacts or with non-family (Bagget et al.; Bi et al.; Burke et al.; Chen Y, Wang, et al.; Cheng HY, Jian et al.; Danis et al.; Ghinai, McPherson et al.; Jing et al.; Li W, Zhang et al.; Yong SEF et al.) In superspreader events, large groups talked, sang, etc. indoors. But doing those activities outdoors, or keeping quiet or speaking softly indoors (such as theaters, class rooms, or offices), did not spread COVID-19 to large numbers (Kay). So, outdoor venues are safer, indoor places with flow-through ventilation are less safe, and indoor places with little ventilation are the least safe (Heil).