What did we know about how the virus spreads in a population?
(by March 2021)
You may wish to read the following pages first in order to better understand this page:
What we knew
What we knew about the origin of COVID-19
What we knew about the structure of the SARS-CoV-2 virion
Click on dark blue words or terms to see their meaning in the glossary.
Presymptomatic vs asymptomatic spread
The SARS-CoV-2 virus can spread from individuals who show no symptoms, whether they are presymptomatic — meaning they will develop symptoms soon — or truly asymptomatic meaning they will remain symptom-free throughout their infection.
Research has confirmed that individuals can be infectious not only while experiencing symptoms but also in the two days preceding symptom onset. In fact, this presymptomatic transmission is quite common. However, there is considerably less certainty about the extent to which truly asymptomatic individuals contribute to the spread of the virus, though it is known to occur.
Additionally, COVID-19 symptoms can be so mild that they go unnoticed or are mistaken for other conditions. This makes it even more challenging to determine how much transmission occurs through asymptomatic cases.
Far more contagious than SARS
COVID-19 spreads easily from person to person, often undetected, which makes containment extremely challenging. In contrast, SARS—though ten to twenty times more deadly for those infected — was successfully contained because it did not spread as stealthily.
Contact tracing and Community Spread
Contact tracing — the investigative process of identifying who is spreading the virus to whom and where — is significantly more challenging with COVID-19. The small Asian nation of Singapore initially succeeded in containing the spread through aggressive contact tracing, allowing daily life to continue almost as normal for a few weeks. Authorities were able to identify infection clusters, isolate confirmed cases, and quarantine those exposed.
However, community spread began after many infected individuals were overlooked in migrant worker dormitories. Despite this setback, contact tracing, when combined with social distancing measures, remains an effective strategy for reducing COVID-19 transmission, regardless of a country’s size.
Containment in China
China decided to seal off Wuhan and nearby cities in an effort to contain the virus. Unfortunately, by the time this measure was implemented, many infected individuals had already traveled to other parts of China and the world, unknowingly spreading the virus.
Despite this initial setback, China managed to contain the outbreak within its borders by mid-April 2020. However, this success came at the cost of nearly three months of strict quarantine and shelter-in-place measures across the country. By mid-May, new cases began to reappear, likely due to incoming travelers bringing the virus back into China.
Number of Infected Individuals Far Greater Than the Case Count in the U.S.
Because many individuals infected with COVID-19 exhibit no symptoms or only mild symptoms, it was nearly impossible during the initial weeks of the pandemic to estimate how many people had actually been infected. Diagnostic tests for COVID-19 first needed to be developed and produced, and widespread testing had to be ramped up.
At the same time, antibody tests — serological tests that detect antibodies in the blood —were developed to help determine how many individuals had previously been infected. Studies using these tests consistently suggested that the true number of infections was far higher than the officially recorded case count. By July 2020, after reviewing mounting evidence, the director of the CDC ventured an educated estimate: the actual number of infections in the U.S. could be ten times higher than the reported case count.
The Implications of a Far Greater Number of Infections Than Known Cases
The growing evidence that actual infections far outnumber reported cases carries both good and bad news.
The bad news is that the virus spreads widely and often undetected, making it much harder for vulnerable individuals to protect themselves than initially believed. With so many asymptomatic or mildly symptomatic cases, containment and prevention efforts face significant challenges.
The good news, however, is that the fatality rate of COVID-19 is lower than it first appeared. Since deaths are measured against a much larger pool of infections than initially known, the overall risk of dying from the virus, once infected, is considerably reduced.
Understanding the True Mortality Rate
The case mortality rate (CFR) refers to the percentage of known COVID-19 cases that result in death. However, this does not reflect the true mortality rate, which would require knowing the total number of infections, including those that were never detected due to mild or asymptomatic cases.
To determine the true mortality rate, widespread testing of an entire population would be necessary—an effort that is costly and time-consuming. Initial estimates suggested a rate above 2%, but as antibody studies revealed far more infections than reported cases, estimates dropped. In the U.S., the true mortality rate could be closer to 0.5%, still significantly higher than the typical flu (which has a mortality rate of about 0.1%).
The actual mortality rate is not a fixed number, as it depends on multiple factors:
- Advances in treatment can lower it.Overburdened hospitals due to lack of social distancing can raise it.
- Higher initial viral loads (from close, prolonged exposure) may lead to more severe disease and worse outcomes.
- Socioeconomic disparities play a role, as individuals in underprivileged communities often face higher exposure risks and less access to medical care.
More testing and data collection remain crucial for refining estimates and guiding public health decisions, including the reopening of schools and businesses.
How COVID-19 Spreads
COVID-19 primarily spreads in two ways:
- Airborne Transmission – The virus spreads when an infected person releases respiratory droplets containing SARS-CoV-2. This can happen through coughing, sneezing, talking, yelling, singing, or even breathing. Inhaling these droplets, especially in crowded or poorly ventilated spaces, increases the risk of infection.
- Surface Transmission – The virus can also spread through surfaces (fomites). If someone touches a contaminated surface that someone else had touched and then touches their nose, mouth, or eyes, they may become infected. While this is a less common mode of transmission, proper hand hygiene is still essential.
Greater transmission through droplets
Scientific evidence increasingly showed that COVID-19 spread more through airborne droplets than through fomites. This is why health authorities, including the CDC, strongly recommended wearing masks — especially indoors or in situations where maintaining physical distance is difficult.
Outdoors, respiratory droplets tend to disperse quickly, reducing the risk of transmission. However, in indoor environments, these droplets can linger in the air for extended periods, increasing the risk of infection.
Additionally, research suggested that under certain conditions, droplets could aerosolize, meaning they become small enough to remain suspended in the air for a far longer period. This makes proper ventilation, mask use, and distancing crucial in enclosed spaces.
(Unfortunately, during the first year of the pandemic, there was not enough conclusive evidence for the scientific community to be certain that aerosols played a major role in transmission. By 2025, however, it is now widely recognized as a scientific fact that aerosol transmission is a significant route of COVID-19 spread.)
Superspreaders
Some individuals, known as superspreaders, infect significantly more people than the average person with COVID-19. One possible explanation is that they expel a higher number of aerosolized droplets, increasing the risk of airborne transmission.
Research suggests that the louder a person speaks, yells, or sings, the more respiratory droplets they produce. This has been observed in documented superspreader events, such as outbreaks in noisy workplaces like meatpacking plants or among choir members singing in enclosed spaces. These cases highlight the importance of masks, proper ventilation, and distancing—especially in environments where people project their voices.
Spreading Through Children
COVID-19 can infect individuals of all ages, including the unborn. However, younger children may be less likely to become infected and to transmit the virus, though the exact extent remains uncertain. What is clear is that the mortality rate for children is extremely low, particularly for those under 15 years of age in the U.S.
How Often Do Children Get Infected?
Studies suggest that children under 10 may be significantly less likely to become documented COVID-19 cases. For example, an analysis of U.S. case data up to May 30 indicated that children under 10 were ten times less likely to be reported as COVID-19 cases compared to the general population. However, this statistic may not fully reflect infection rates, as children are tested less frequently due to their lower likelihood of developing symptoms or requiring hospitalization.
In Austria, an analysis of infection rates found that children aged 5 to 14 years were infected at about 40% of the rate of adults aged 55 to 64. This suggests that adults in this age range were only about 2.5 times more likely to be infected than school-age children. These figures, however, may be affected by differences in testing rates and age groupings, as younger children (under five) may have even lower infection rates than older school-age children.
Can Children Spread COVID-19?
The likelihood of children transmitting COVID-19 appears to depend on age. Young children (0–9 years old) are much less likely to spread the virus to adults, though the risk is not zero. According to CDC-supported contact tracing studies from South Korea, transmission from young children to adults is relatively rare. However, other studies indicate that even young children can, in some cases, pass the virus on.
In contrast, older children and teenagers (10–19 years old) may transmit COVID-19 as readily as adults. This has significant implications for school reopenings and public health measures.
Understanding these differences is crucial for developing effective policies to protect both children and the broader community.
Spreading through other mammals
COVID-19 is not limited to humans—several mammal species have also tested positive for the virus. Since humans originally contracted SARS-CoV-2 from another species, it is also possible for us to transmit it to other mammals.
So far, dogs, cats, tigers, lions, and mink have been confirmed to carry the virus. Notably, farmed mink in the Netherlands appeared to have transmitted COVID-19 back to humans, making them the first known case of animal-to-human reinfection. However, there is no strong evidence of similar transmission from other mammals.
Continued research is essential to identify which species may act as reservoirs for the virus and whether they could contribute to future outbreaks. Understanding these risks is crucial for mitigation and containment efforts, especially in environments where humans and animals interact closely.
Prevention and social distancing
Following recommended prevention measures (see the Prevention pages on this site), including social distancing, is crucial in slowing the spread of COVID-19. By maintaining physical distance, we can help:
- Prevent hospitals from becoming overwhelmed, ensuring that those who need medical care receive it, which helps keep the mortality rate low.
- Give scientists time to develop better treatments, including more effective medications for COVID-19.
- Allow for vaccine development to protect communities before more lives are lost needlessly.
Social distancing, along with other preventive actions, plays a vital role in managing the pandemic and protecting public health.
Continue with What we knew about how the virus causes COVID-19 disease.
©2020, 2021 Dr. Michael Herrera