Infectious before symptoms

It is now completely clear that the virus that causes Covid-19, SARS-CoV-2, is transmitted not only from people who have symptoms but also from those who don’t have symptoms yet (presymptomatic) and even those who will never show symptoms (truly asymptomatic). Studies show that most viruses are excreted the day before and the day after the onset of symptoms, and that you can be contagious for several days before that. With the Delta viariant, maximum excretion is earlier, according to some data around two days before symptoms. Different compilations come to different conclusions about the proportion of the spread of infection that occurs from pre-/asymptomatic individuals, but often the stated figure is close to 50 percent (see below). One of the contributing reasons is probably that a person who has no symptoms is less careful in his dealings with others. With this in mind, the recommendation "stay home if you feel sick" is completely inadequate.

Pre-/asymptomatic infection is one of the main reasons why you should use facemasks in public indoor environments or when distances cannot be maintained.

Transmission reports (infector-infectee pairs)

Kang, et al., “Transmission dynamics and epidemiological characteristics of Delta variant infections in China”
Finding: A study of 167 Delta cases, and 94 transmission pairs, in Guangdong province. Infections with the Delta variant differed significantly from the original wild-type infections. Delta viral load was higher, transmissibility was higher, and, significantly, transmission not only started, but peaked, days before symptom onset. 2.7% of transmission occurred 7 days before illness onset, 22.5% occurred 4 days before illness onset, and the peak of infectiousness was reached 2.1 days before illness onset. After this infectiousness gradually decreased. In all, 73.9% of transmission occurred before symptom onset.

Bender, et al., “Analysis of Asymptomatic and Presymptomatic Transmission in SARS-CoV-2 Outbreak, Germany, 2020”
Emerging Infectious Diseases, April 2021
Finding: In a German rural district, 254 close contacts of 59 symptomatic and a-/pre-symptomatic Covid-19 patients were followed. Transmission rate was 14%. More than 75% of transmission occurred from pre-symptomatic patients.

Johansson, et al., “SARS-CoV-2 Transmission From People Without COVID-19 Symptoms”
JAMA Network, January 2021
Finding: Based on data from a meta-analysis of 8 studies from China, a decision analytical model determined that SARS-CoV-2 transmission from asymptomatic individuals accounted for more than half of all transmission.

Casey-Bryars, et al., “Presymptomatic transmission of SARS-CoV-2 infection: a secondary analysis using published data”
BMJ Open, May 2021
Finding: A meta-analysis of data from December 2019 to May 2020. The proportion of presymptomatic transmission ranged from 45.9% to 69.1%

Ding, et al., “Estimating the time interval between transmission generations and the presymptomatic period by contact tracing surveillance data from 31 provinces in the mainland of China”
Fundamental Research, March 2021
Finding: A collection of 3453 infector-infectee pairs from 31 Chinese provinces. The mean presymptomatic period of Covid-19 was estimated to be 2.4 days.

Sun, et al.: “Transmission heterogeneities, kinetics, and controllability of SARS-CoV-2”
Finding: Calculation of transmission risk between 1,178 SARS-CoV-2 infected individuals and their 15,648 close contacts based on detailed contact tracing data from Hunan, China. Finding that 80% of secondary transmissions can be traced back to 14% of SARS-CoV-2 infections, indicating substantial transmission heterogeneities. The reconstructed infectiousness profile of a typical SARS-CoV-2 infection peaks just before symptom presentation, with ~50% of transmission occurring in the pre-symptomatic phase.

Du, et al.: “Serial Interval of COVID-19 among Publicly Reported Confirmed Cases”
Emerging infectious Diseases, June 2020
Finding: 468 transmission events (infector-infectee pairs) were studied. Mean serial interval: 3,96 days. Serial interval was shorter than incubation period, indicating presymptomatic transmission. In addition: In 59 pairs (12.6% of total), serial interval was negative, and infectee developed symptoms before infector (i.e. infectee went through entire incubation period, and developed symptoms, before infector developed symptoms)

Ma, et al., ”Epidemiological parameters of coronavirus disease 2019: a pooled analysis of publicly reported individual data of 1155 cases from seven countries”
LINK to Preprint
Finding: Data from 10,728 reports. 1,155 cases were included, from various countries. The time point of exposure was estimated from 233 pairs of infector-infectee. In 43.78% of infector- infectee pairs, transmission occurred before onset of symptoms in infector. In 3.92% of infector-infectee pairs, infectees’ symptom onset preceded that of infectors’.

Ferretti, et al., “Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing”
Science, May 8, 2020
Finding: 40 well-characterized infector-infectee pairs were analyzed. The authors estimated that 46% of transmission was from presymptomatic individuals, 38% was from symptomatic individuals, 10% was from asymptomatic (never symptomatic) individuals, and 6% from environmentally mediated transmission via contamination (the last two routes were speculative). “According to these estimates, presymptomatic transmissions alone are almost sufficient to sustain epidemic growth.”

Nishiura, et al: “Serial interval of novel coronavirus (2019-nCoV) infections” Preprint
Finding: 28 infector-infectee pairs. Mean serial interval of 4.0-4.6 days. Serial interval shorter than incubation period, indicating substantial pre-symptomatic transmission. However, serial interval not negative among these pairs.

Zhao, et al.: “Estimating the serial interval of the novel coronavirus disease (COVID-19): A statistical analysis using the public data in Hong Kong from January 16 to February 15, 2020.”
Finding: 21 infector-infectee pairs. Mean serial interval of 4.4 days. Serial interval shorter than incubation period, indicating substantial pre-symptomatic transmission. However, serial interval not negative among these pairs.

Furuse, et al., “Clusters of Coronavirus Disease in Communities, Japan, January–April 2020”
Emerging Infectious diseases, sept 2020
Finding: 61 case-clusters were identified in Japan, with 22 probable primary case-patients for the clusters. Most were 20–39 years of age and presymptomatic or asymptomatic at virus transmission.

Wei, et al: “Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020”
CDC Morbidity and Mortality Weekly Report (MMWR), April 1, 2020
Finding: Investigation of all 243 cases of COVID-19 reported in Singapore during January 23–March 16 identified seven clusters of cases in which pre-symptomatic transmission is the most likely explanation for the occurrence of secondary cases (6.4% of locally acquired cases).

Ganyani, et al: “Estimating the generation interval for COVID-19 based on symptom onset data”
Finding: A modelling study, based on clusters in Singapore and Tianjin, China. The proportion of pre-symptomatic transmission was estimated at 48% for Singapore, and 62% for Tianjin.

Li, et al.: “Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2).”
Science, March 16, 2020
Finding: A study on the fractions of undocumented cases in China, and their contagiousness. The authors estimate that 86% of all infections were undocumented prior to the January 23 travel restrictions. “Per person, the transmission rate of undocumented infections was 55% of documented infections ([46%–62%]), yet, due to their greater numbers, undocumented infections were the infection source for 79% of documented cases.”

Cheng, et al., “High transmissibility of COVID-19 near symptom onset” Preprint
Finding: Thirty-two confirmed patients were enrolled and 12 paired data (index-secondary cases) were identified among the 1,043 contacts. The secondary clinical attack rate was 0.9%.
The attack rate was higher among those whose exposure to index cases started within five days of symptom onset than those who were exposed later.

Transmission from and incidence of truly asymptomatic cases

Qiu, et al., “The role of asymptomatic and pre-symptomatic infection in SARS-CoV-2 transmission—a living systematic review”
Clinical Microbiology and Infection, January 2021
Finding: A metaanalysis of 80 studies, concerning transmission of SARS-CoV-2 from consistently asymptomatic, presymptomatic and symptomatic individuals. The authors conclude that consistently asymptomatic individuals can transmit the virus, but are responsible for fewer secondary infections than pre-symptomatic or symptomatic individuals.

Krieg, et al., “Symptomatic, Presymptomatic, and Asymptomatic Transmission of SARS-CoV-2
Finding:Study based on a cohort of 12,960 young adults. A third of the 1,556 students who became virus positive did not develop symptoms, within 15 days of the positive test. Transmission rate for those asymptomatic individuals was 19.1%, whereas transmission rate from symptomatic cases was only slightly higher, at 25.4%.

Wu, et al., “Assessing Asymptomatic, Presymptomatic, and Symptomatic Transmission Risk of Severe Acute Respiratory Syndrome Coronavirus 2”
Clinical Infectious Diseases, March 2021
Finding:  A study of 393 symptomatic index cases with 3136 close contacts and 185 asymptomatic index cases with 1078 close contacts. The secondary attack rates among close contacts of symptomatic and asymptomatic index cases were 4.1%  and 1.1%, respectively, corresponding to a higher transmission risk from symptomatic cases than from asymptomatic cases.

Sah, et al., “Asymptomatic SARS-CoV-2 infection: A systematic review and meta-analysis”
PNAS, August 2021
Finding: An analysis of 350 studies, concluding that 35.1% of SARS-CoC-2 infections were truly asymptomatic, i.e. never developed symptoms.

Alene, et al., “Magnitude of asymptomatic COVID-19 cases throughout the course of infection: A systematic review and meta-analysis”
PLOS ONE, March 2021
Finding: A meta-analysis finding that one fourth of SARS-CoV-2 infections remain asymptomatic.

Oran and Topol, “The Proportion of SARS-CoV-2 Infections that are Asymptomatic”
Annals of Internal Medicine, May 2021
Finding: Available data suggest that at least one third of SARS-CoV-2 infections are asymptomatic. Longitudinal studies suggest that nearly three quarters of persons who receive a positive PCR test result but have no symptoms at the time of testing will remain asymptomatic.

He, et al., “Proportion of asymptomatic coronavirus disease 2019: A systematic review and meta-analysis”
Journal of Medical Virology, July 2020
Review of truly asymptomatic cases

Family clusters

Yu, et al.: “A Familial Cluster of Infection Associated With the 2019 Novel Coronavirus Indicating Possible Person-to-Person Transmission During the Incubation Period.”
Journal of Infectious Diseases, Feb 18, 2020
Finding: Early report of familial cluster in China, where two people in Shangai were infected by one of two presymptomatic visitors from Shanghai. No other risk factors.

Pan, et al: “Asymptomatic cases in a family cluster with SARS-CoV-2 infection”
Lancet, Feb 19, 2020
Finding: Early report of a family with both symptomatic and asymptomatic infected members.

Huang, et al: “A family cluster of SARS-CoV-2 infection involving 11 patients in Nanjing, China”
Lancet, Feb 28, 2020
Finding: Early report of series of pre-symptomatic transmissions within family cluster in China.

Huang, et al.: “Rapid asymptomatic transmission of COVID-19 during the incubation period demonstrating strong infectivity in a cluster of youngsters aged 16-23 years outside Wuhan and characteristics of young patients with COVID-19: A prospective contact-tracing study.”
Journal of Infection, April 10, 2020
Finding: Strong infectivity in a cluster of presymptomatic youth in China, outside Wuhan.

Tong, et al.: “Potential Presymptomatic Transmission of SARS-CoV-2, Zhejiang Province, China, 2020”
Emerging Infectious Diseases, May 2020
Finding: 2-family cluster infected by likely pre-symptomatic patient.

Bai, et al., “Presumed Asymptomatic Carrier Transmission of COVID-19” JAMA, February 21, 2020
Finding: Familiar cluster of five patients, infected by asymptomatic carrier travelling from Wuhan.

Hu, et al., “Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China”
Science China. Life Sciences, March 4, 2020
Finding: Clinical characteristics of 24 asymptomatic patients, where some transmitted the infection to others.

Qian, et al.: “A COVID-19 Transmission within a family cluster by presymptomatic infectors in China”
Clinical Infectious Diseases, March 23,2020
Finding: Family cluster where 8 out of 9 family members were infected by pre-symptomatic case (and two remained asymptomatic).

Madewell, et al.: “Household Transmission of SARS-CoV-2. A Systematic Review and Meta-analysis”
JAMA Network Open, December 20, 2020
Finding: In this meta-analysis of articles on household transmission of SARS-coronavirus-2 (SARS-CoV-2), the authors claim that infected symptomatic individuals are 25 times more likely to spread the disease than infected individuals without symptoms (pre- or asymptomatic). However, in the four studies cited, three in fact did not compare the secondary spread from symptomatic individuals with that from a- or presymptomatic individuals (references 43, 44 and 52). The first study (reference 43) reported that one person presymptomatically infected one of 23 close contacts (4%), and the two other articles reported on 2 and 4 asymptomatics that did not infect any of their close contacts (references 44 and 52). In the study that actually compared secondary spread from a- or presymptomatic individuals with that from symptomatic individuals, the difference was only 1.27 times (Chaw, et al, “Analysis of SARS-CoV-2 Transmission in Different Settings, Brunei”, Emerging Infectious Diseases, November, 2020 This last article studied the transmission of SARS-CoV-2 from 66 infected individuals to close contacts. Of the 66 infected individuals, 36 were symptomatic, 21 were presymptomatic and 9 were asymptomatic. Twenty of them transmitted the infection to their close contacts. Of these twenty spreaders, five were presymptomatic, two were asymptomatic and thirteen had symptoms. The seven pre- or asymptomatics infected 15 of their 691 close contacts (2.2%) and the thirteen symptomatics infected 28 of their 1010 close contacts (2.8%). Hence, the symptomatic individuals were 1.27 times, not 25 times, more infectious than pre- or asymptomatic individuals.

Virus isolation – symptomatic and a/pre-symptomatic

Lee, et al.: “Clinical Course and Molecular Viral Shedding Among Asymptomatic and Symptomatic Patients With SARS-CoV-2 Infection in a Community Treatment Center in the Republic of Korea”
JAMA, August 6, 2020
Finding: Study of 303 COVID-19 isolated patients in a community treatment center in South Korea. 110 were asymptomatic at the time of isolation, and of these, 21 developed symptoms during the isolation. Viral loads in asymptomatic patients was similar to that in symptomatic patients.
Additional findings: The median interval of time from detection of SARS-CoV-2 to symptom onset in presymptomatic patients was 15 (13-20) days. The median time from diagnosis to the first negative conversion was 17 days for asymptomatic patients and 19.5 days for symptomatic (including presymptomatic) patients (P = .07).

He, et al.: “Temporal dynamics in viral shedding and transmissibility of COVID-19”
Nature Medicine, April 15, 2020
Finding: Temporal patterns of viral shedding in 94 laboratory confirmed cases of covid-19, from day of symptom onset up to 32 days after (414 throat swabs in total). Viral shedding was highest on the day of onset and gradually decreased. Separately, the authors collected data on 77 transmission pairs from publicly available sources, and calculated statistically that infectiousness on average starts 2,5 days before onset of symptoms, and peaks 0,6 days before onset of symptoms. The proportion of pre- symptomatic transmission they estimated at 44%.

To, et al.: “Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study”
Lancet, March 23, 2020
Finding: 23 symptomatic patients tested with PCR. Viral load was highest during the first week after symptom onset, and then declined with time (in one patient RNA detected 25 days after symptom onset).

Sutton, et al.: “Universal Screening for SARS-CoV-2 in Women Admitted for Delivery” New England Journal of Medicine, April 13, 2020
Finding: 215 pregnant women delivered babies at two New York City medical centers between March 22 and April 4, 2020. Of these, 214 were screened for SARS-CoV-2 (nasopharyngeal swabs). Four women (1,9%) had symptoms of Covid-19, and all four were positive for the virus. Of the remaining 210 symptom-free women, 29 (13,7%) were found positive for the virus. Thus, 87,9% of the SARS-CoV-2-positive women were asymptomatic. Of these, three women developed fever before postpartum discharge.

Cereda, et al.: “The early phase of the COVID-19 outbreak in Lombardy, Italy”
Finding: Analysis of the first 5,830 laboratory-confirmed cases reported in Lombardy, Italy, with date of symptom onset. Equal viral loads were found in nasal swabs from symptomatic and asymptomatic cases.

Lavezzo, et al.: “Suppression of COVID-19 outbreak in the municipality of Vo, Italy”.
Finding: Two surveys (nasopharyngeal swabs) of most of the population of the municipality of Vo, in Italy. Of those who tested positive (2.6% and 1.2%, respectively, of the tested population), 43.2% were found to be asymptomatic. Viral load was equal in symptomatic and asymptomatic cases. Most new infections in the second survey were infected in the community before the lockdown or from asymptomatic infections living in the same household.

Kimball, et al.: “Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility — King County, Washington, March 2020.” CDC Morbidity and Mortality Weekly Report (MMWR), April 3, 2020
Finding: Report on rapid spread in a long-term care nursing facility. Testing of 76 of 82 residents found that 30.3% shed virus, but only half of those infected had symptoms.

Baggett, et al.: “COVID-19 outbreak at a large homeless shelter in Boston: Implications for universal testing”
Finding: Of 408 participants in a homeless shelter in Boston, 147 (36.0%) were PCR- positive for SARS-CoV-2. Around 80% were asymptomatic.

Liu et al., “Viral dynamics in mild and severe cases of COVID-19”
Lancet, March 19, 2020
Finding: 76 patients with COVID-19 were admitted to a hospital in Nanchang, China, and tested for viral loads with nasopharyngeal swabs. “The mean viral load of severe cases was around 60 times higher than that of mild cases, suggesting that higher viral loads might be associated with severe clinical outcomes.”

Arons, et al.: “Presymptomatic SARS-CoV-2 Infections and Transmission in a Skilled Nursing Facility”
New England Journal of Medicine, April 24, 2020
Finding: A follow-up study in a nursing facility. Twenty-three days after the first positive test result, 57 of 89 residents (67%) tested positive for SARS-CoV-2 (nasopharyngeal and oropharyngeal swabs). Of these, 27 (56%) were asymptomatic at time of testing, but 24 of them subsequently developed symptoms (median time to onset, 4 days). Viral loads were similarly high in the four symptom groups (residents with typical symptoms, those with atypical symptoms, those who were presymptomatic, and those who remained asymptomatic). It is notable that 17 of 24 specimens (71%) from presymptomatic persons had viable virus by culture 1 to 6 days before the development of symptoms.

Gandhi, et al.: “Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19”
New England Journal of Medicine, April 24, 2020 Editorial.

Zou, et al: “SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients” New England Journal of Medicine, Feb 19,2020
Finding: Analysis of viral load in nasal and throat swabs from 17 patients, taken daily. Viral load similar in asymptomatic and symptomatic patients. In symptomatic patients, viral load highest at time of symptom onset.

Nishiura, et al: “Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19)”
International Journal of Infectious Diseases (preproof)
Finding: Among Japanese citizens evacuated from Wuhan by February 6, the asymptomatic ratio was estimated at 30.8%.

Comprehensive testing on Iceland (news report): Finding: On Iceland, 17,900 people have been broadly tested, in two studies. A study of 9,000 people who volunteered to be tested (deCode) found that 1% were shedding virus. Only 50% of them were symptomatic. The other study, based in the National University Hospital, tests cases and their contacts. In that study, about 80% of infected people show symptoms.

Gudbjartsson, et al.: “Spread of SARS-CoV-2 in the Icelandic Population” New England Journal of Medicine, April 14, 2020
Finding: 6% of the population of Iceland was screened (nasopharyngeal and oropharyngeal samples). Of those tested in the general population, 0.8% tested positive for the virus. Of those, 43% were asymptomatic. Children under 10 years of age and females had a lower incidence of SARS-CoV-2 infection than adolescents or adults and males.

Statista, State of Health: “Age distribution of coronavirus (COVID-19) cases in South Korea as of April 3, 2020” (daily update)
Finding: In South Korea, where there is broad testing, the highest proportion of positive cases is found in the age group 20-29 years (27,17%). From this data, it appears that the ratio of asymptomatic cases is dramatically higher in young adults.

Mizumoto, et al., “Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020”
Eurosurveillance, March 2020
Finding: Estimation of true asymptomatics on cruise ship Diamond Princess. The authors arrive at the figure 17.9%. The majority of passengers were in their 60s and 70s.

Moriarty, et al.: “Public Health Responses to COVID-19 Outbreaks on Cruise Ships — Worldwide, February–March 2020”
CDC Morbidity and Mortality Weekly Report (MMWR), March 27, 2020
Finding: Among 3,711 Diamond Princess passengers and crew, 712 (19.2%) had positive test results for SARS-CoV-2. Of these, 331 (46.5%) were asymptomatic at the time of testing.
Among 381 symptomatic patients, 37 (9.7%) required intensive care, and nine (1.3%) died.

Koh, et al., “What do we know about SARS-CoV-2 transmission? A systematic review and meta-analysis of the secondary attack rate, serial interval, and asymptomatic infection”
Finding: “Based our pooled estimates, 10 infected symptomatic persons living with 100 contacts would result in 15 additional cases in less than 5 days. To be effective, quarantine of contacts should occur within 3 days of symptom onset. If testing and tracing relies on symptoms, one-quarter of cases would be missed.”

Rasmussen and Popescu, “SARS-CoV-2 transmission without symptoms”
Science, March 2021

Han, et al.: “The transmission and diagnosis of 2019 novel coronavirus infection disease
(COVID‐19): A Chinese perspective”
J. Med. Virol. March 6, 2020

Aguirre-Duarte: “Can people with asymptomatic or pre-symptomatic COVID-19 infect others: a systematic review of primary data”

Oran, et al.: “Getting a handle on asymptomatic SARS-CoV-2 infection”
Scripps Research, April 20, 2020 infection/index.html

Oran et al., “Prevalence of Asymptomatic SARS-CoV-2 Infection” Annals of Internal Medicine, June 3, 2020

Apuzzo, et al., “How the World missed Covid-19s Silent Spread”
New York Times
Review of initial public health reactions.

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Science Forum Covid-19

Science forum Covid-19 works extensively with information and information about the ongoing pandemic and related topics and areas. Our goals are to reduce morbidity and mortality in Covid-19, to reduce the chronic disease states of those who survive Covid-19. In the association, we have expertise in areas such as virology, biology, molecular biology, epidemiology, infectious diseases, lung diseases, mathematics, political science, psychology, ethics and risk research. The chairman of the association is Professor Emeritus Anders Vahlne, more information at