Risky Situations

Asymptomatic and Presymptomatic Transmission

[Date of latest publication cited: November 20, 2020]

People with SARS-C0V-2 infection, but without symptoms, probably cause much of the transmission to others., especially in households (Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Lee E, Wada, et al.).  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.; Lee S, Kim, 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 example, in the heavily infected Italian town of Vo, testing most of the population twice found about 40% of infected people had no symptoms, and the symptomatic and asymptomatic cases had about equal viral loads that could transmit to others (Imperial College London; Lavezzo et al.).  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.).  Among people found infected in a program in Korea, 36% were asymptomatic.  Then 19% started having symptoms, showing that they had actually been presymtomatic (Lee S, Kim, 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 Y, Li X 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…”).

Isolation and quarantine

[Date of latest publication cited: November 19, 2020]

Since many infected people are asymptomatic or presymptomatic, as described above, they should isolate themselves from others (Lee S, Kim, et al.).  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 (Cevik, Tate, et al.), 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).  So, people who have been infected should isolate from other people for at least 10 days after symptoms started, and their symptoms improved, and they have not had a fever for 24 hours without using fever reducing medicines.  After meeting all those conditions, they can come out from isolation   (Centers for Disease Control “Duration of Isolation”; Centers for Disease Control “Symptom based strategy”; Gurley; Sethuraman, Jeremiah.).

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 (Centers for Disease Control “When to Quarantine”; Centers for Disease Control “Symptom based strategy”; Gurley; Sethuraman, Jeremiah.).

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.

These patterns of symptoms, RNA, and viruses could allow many asymptomatic and presymptomatic 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 (Clipman et al.).

Contact tracing of COVID-19 transmission has succeeded in reducing it.  For example, in some fitness centers in Korea, contact tracing and social distancing stopped an outbreak (Bae, Kim, et al.).


[Date of latest publication cited: November 19, 2020]

After people have COVID-19, most probably become immune, and most probably cannot get infected again (Lumley et al. “Antibodies to SARS-CoV-2”).  For example, on a fishing boat, three crew members had antibodies.  During their next voyage, one other person infected most of the others, but not the three already having antibodies (Addetia et al.; Tingley).

Many convalescent patients developed SARS-CoV-2 neutralizing antibodies, IgM, IgG, and IgA which decreased months later ( Allegra et al.; Choe et al; Klar; Ledford; Lei et al.; Long et al.; Lowe; Lumley et al.”Duration, dynamics and determinants of SARS-CoV-2”; Reifer et al.; Rodda et al.; Seow et al.; Wajnberg et al.; Wu F, Liu, et al.).  One study showed that among healthcare workers with antibodies, none developed symptomatic infection in the next 6 months (Lumley eta l. “Antibodies to SARS-CoV-2”). A study of patients for 6 – 8 months found spike IgG antibodies were stable, spike-specific memory B cells increased from 1 to 6 months, and CD4+ T cells and CD8+ declined (Dan et al.; Miller).  Two follow-up surveys of infected people found 90% had antibodies for at least four to six months (Figueiredo‐Campos et al.; Gudbjartsson et al.; Paulsen).  Memory T cells, CD4+ helper T cells, CD8+ killer T cells, and memory B cells, which are kinds of white cell lymphocytes, develop and provide immunity for as long as six months (Allegra et al.; Calhoun et al.; Grifoni et al.; Kelland “T-Cell Study”; Ledford; Lewis R; Lowe; Mandavilli “Can you get COVID-19 again?”; Mateus et al.; Ni et al.; Resnick, Irfan; Robbiani et al.; Sekine et al.; Thieme et al.; Yong “Immunology”; Zuo et al.).

Some people who probably had previous coronaviruses, such as SARS, MERS, and common colds, developed T cells which could attack SARS-CoV-2 (Le Bert et al.).  Consequently, patients with no detectable antibodies might still be immune, because the T cells, B cells, and small amount of antibodies could prevent infection (Gorvett; Iwasaki, Medzhitov; Resnick, Irfan; Sette, Crotty).  Altogether this indicates that future vaccines will probably effectively immunize many people (Lowe).  Patients’ antibodies, T cells, and B cells varied, perhaps indicating that disease-induced, and vaccine-induced immunity among large populations will probably vary (Juno et al.; Walker “Study”; “Wide range“; Wu F, Liu, et al.).  But scientists do not yet know how long immunity will last (Centers for Disease Control “Duration of Isolation” and “Updated Isolation Guidance”; Ledford “What the immune response”).

In contrast, scientists proved that several people were infected a second time with a genetically different strain of SARS-CoV-2.  Most of them had more serious symptoms the first time, and less serious or no symptoms the second time.  This may show that most people’s immune systems can defend them from reinfection, and that reinfections might cause mild or no symptoms in some people (Iwasaki; Mandavilli “Coronavirus reinfections”; Marchione; Pieters; Regalado; Spinks; To KKW, Hung, et al.; Van Elslande et al.).  But, in some cases the second infection caused more serious symptoms than the first, even causing death, showing that the immune system did not protect them thoroughly (BNO News “Dutch researchers report first death”; BNO News “COVID-19 reinfection tracker”Bowden; Hersher; Mulder et al.; Prado-Vivar et al.; Tillett et al.).  More scientists are developing methods to test large numbers to find if they have viral relapse, continuing infections,  reinfections, or inflammatory rebounds.  Then they can estimate the incidence of re-infections (Gousseff et alLedford “Coronavirus reinfections”; Murillo-Zamora et al; Tomassini et al; Yeager).   So, SARS-CoV-2 could continue circulating for years, even if people have infection-induced or vaccine-induced immunity, similar to the annual epidemics of common colds and influenza (Walker “First case”).

Some patients had COVID-19 symptoms for months, most testing positive for viral RNA, and some not (Alwan; Carfi et al.; Garner; Malta; Mandavilli “Can you get COVID-19 again?”; Marshall; Rubin; Tuller; Yong “COVID-19 can last..”and “Long haulers“; Witvliet). Some apparently recovered, had no symptoms or viral RNA, and then showed RNA again later.  Some of those also had antibodies (Katz; Liotti et al.).  Some had no symptoms or viral RNA, and then showed symptoms and RNA again later. They will probably need more medical treatments, and research on improving treatments and self-care.  These probably were continuing the same disease case, and were not reinfected.  More follow-up studies could discover if they had the same or continuing infections (Yong “Immunology”).  Follow-up found none transmitting to contacts (Korea Centers for Disease Control; Mandavilli “Can you get COVID-19 again?” ) and none having viruses that could infect cells (Lu J, Peng J et al.; Simon).  Their long term symptoms might be caused by their immune systems reacting against their own internal organs, creating autoreactive antibodies or autoantibodies (Mandavilli “Some Covid Survivors”; Woodruff et al.).  These varied disease patterns can be classified in three categories: “Acute Infection or COVID-19;” “Postacute Hyperinflammatory Illness;” and “Late Inflammatory and Virological Sequelae” (Datta et al.).

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 J, 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.

Herd immunity

[Date of latest publication cited:  November 3, 2020]

Biological herd immunity occurs when most people develop immunity to the disease pathogen, either from natural infection or vaccination.  Some experts hypothetically estimated that perhaps if 43% – 80% of a population are immune to SARS-CoV-2, then it could not transmit through the whole population (Clemente-Suárez; Brumfiel; Hartnett; Mandavilli “Can you get COVID-19 again?”; Mandavilli “What if heard immunity”; Randolph, Barreiro; Resnick, Irfan).  A study of blood plasma collected from urgent care and routine care patients of a New York City hospital discovered that 20% of the people have antibodies, only a fraction of the number needed for herd immunity (NBC; Stadlbauer et al.).  A survey of hospital patients with diseases other than COVID-19 in Wuhan in March-May found 3.9% had antibodies to SARS-CoV-2, which indicates that even that highly-infected city is probably not near herd immunity (Liu A, Li Y, et al).  A survey of US dialysis patients’ blood samples in July found about 9% infected, ranging from 3.5% in the west to 27% in the northeast (Anand et al.)  So, Europeans and Americans had been infected at rates only a fraction of those needed to create herd immunity.  In comparison, six months into one of the worst outbreaks, in San Quentin prison, over 2/3 of San Quentin prison inmates were infected, and hundreds died, yet the epidemic is still spreading there.  This shows that herd immunity probably needs more than 2/3 immune (Lin, Christensen).  A review of European COVID-19 statistics in June 2020 found that they were not near herd immunity, and that social distancing and other interventions caused the epidemics to plateau at that time (Okell et al.).

Some scientists advocate allowing healthy members of the public to live as they did before the pandemic, get infected, and develop herd immunity, while protecting more vulnerable people from infection (Kuldorf et al.)  Others disagreed, saying this would cause much suffering; it is not feasible to identify, isolate, and protect large numbers of vulnerable people; and many public health measures would better protect people (Alwan, Burgess, et al.; Gronvall; The John Snow Memorandum; Walker “Researchers blast”).

Social herd immunity occurs when most people know someone who got sick with that infectious disease, and talk to each other about effective prevention methods.  The large numbers of people who deny the seriousness of this disease or, resist using scientifically valid COVID-19 prevention methods, shows that many communities have not developed social herd immunity to this disease (Fleming; The Lancet “The Truth is Out There”; Lewis T).

Exhaling indoors for hours

[Date of latest publication cited: November 20, 2020]

Probably much COVID-19 transmission occurs indoors (Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”). 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; Furuse et al.; Huang “Why the Coronavirus”; Huang “Researchers Say Fresh Air”; Leclerc et al.; Qian H, Miao, et al.).   One study estimated the probably of getting COVID-19 as 18.7 times higher in a closed environment than an open environment ( Nishiura, Oshitani, et al.).  The previous section described patients transmitting to their families at home.  In other episodes, people were breathing, coughing, sneezing, yelling, and touching each other (Asadi, Wexler, et al.; Atkinson et al.; Furuse et al.; National Academies “Airborne Transmission of SARS-CoV-2” Tang JW, Li et al.) in restaurants (Li Y, Qian, et al.; Lu J, Gu et al.), a telephone call center (Park et al.), choir rehearsal (Hamner et al.; Read), indoor sports (Dawson), family gatherings (Bromage; Ghinai, Woods et al.), a meat packing plant (Guenther), restaurants and bars (Adam et al.), a nursing facility that recycled air in rooms with little outside air entering (de Man et al.), an international business conference (Lemieux et al.), fitness centers (Bae, Kim, et al.; Jang et al.) a squash court (Briek et al.), and barracks-style migrant worker dormitories (Gorny 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 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.; Ng 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).

Generally, outdoor venues are safer, indoor places with flow-through ventilation are less safe, and indoor places with little ventilation are the least safe (Khazan; Heil). Studies of cell phone mobility comparing US counties and census block groups found that more travel to less-essential businesses (restaurants, fitness centers, and hotels) was associated with higher transmission (Chang S, Pierson et al.; Children’s Hospital; Cooney “Restaurants and gyms”; Dockrill; Guarino, Achenbach; Rubin et al.).  A study comparing COVID-19 outpatients to other matched control-participants found that eating in restaurants was the variable most associated with infection (Edwards; Fisher et al.).  In order to help those businesses continue financially, and gain their benefits, many people are buying their products and services safely.  For example, they ate restaurant and bar food and drinks outdoors, or took it home, or exercised outside gyms, or paid for cooking, nutrition, or exercise lessons from restaurant, bar, and gym personnel.

During winter, more people will congregate indoors.  Cold winter air is often lower humidity than warm summer air, and heating it further reduces humidity, enabling SARS-CoV-2 laden droplets to dry, and float longer.  This might increase COVID-19 transmission.  (Ahlawat et al.; Allen et al.; Bazant, Bush; Cooney “Covid-19’s wintry mix”; Department of Homeland Security; Freedman; Mallapaty; National Academies of Science “SARS-CoV-2 survival…”; Science Daily; Ward et al.).  The lipid envelopes of enveloped viruses (such as influenza and probably SARS-CoV-2) solidify at low temperatures, and can last longer before being inactivated.  Colder temperatures and lower humidity impair the cilia (hairlike covering) and mucous clearing foreign particles from the airways (Hassad; Moriyama et al.) and antiviral defense and tissue repair (Allen et al.; Kudo et al.).

During the recent southern hemisphere winter, influenza cases decreased to less than usual numbers, perhaps because of decreased testing or increased COVID-19 prevention.  So, in this next northern hemisphere winter, people could increase or decrease influenza, depending on their prevention behaviors (Burki “Double Threat”).

To reduce indoor transmission, many people have been opening windows and pumping more outside air into buildings (de Man et al.; Hernandez; Murphy; Tufekci).  Increasing indoor humidity by using humidifiers could reduce COVID-19 transmission (Ahlawat; Allen et al; Bazant, Bush; Science Daily).  Some experts recommended methods for using school buildings so students and teachers can both learn and prevent COVID-19 transmission, as part of an overall healthy buildings program (Harvard TH Chan School of Public Health “For Health, Healthy Buildings”; Jones; Powell).  Others are attempting to reduce these problems by setting up tents or plastic bubbles outside restaurants in which air flows more than indoors, customers wear masks when not eating, and staff clean often (Chiu “Dining bubbles”).

To help people understand and prevent airborne transmission risks, experts developed an online spreadsheet in which one can enter information on the characteristics of a place, and the spreadsheet will estimate numbers of people who could get infected (Jimenez).  Another model estimates safe limits on “cumulative exposure time” indoors, depending on the number people, time, ventilation, room dimensions, breathing rate, respiratory activity, face-mask use, and respiratory aerosols infectiousness (Bazant; Bazant, Bush).