Is It Common to Have COVID-19 Without Getting Symptoms?

Check out this answer from Consensus:

Asymptomatic COVID-19 infections are relatively common and play a significant role in the transmission of the virus. Despite the lack of symptoms, many asymptomatic individuals exhibit clinical and radiological abnormalities. Public health strategies must account for the presence of asymptomatic carriers to effectively control the spread of COVID-19.

The COVID-19 pandemic has posed significant challenges to global health systems, not only due to the severity of symptoms in some patients but also because of the presence of asymptomatic carriers. Asymptomatic COVID-19 infections are those where individuals test positive for the virus but do not exhibit any symptoms. Understanding the prevalence and implications of asymptomatic infections is crucial for effective public health strategies.

Prevalence of Asymptomatic COVID-19 Infections

Several studies have investigated the proportion of COVID-19 cases that are asymptomatic. A systematic review and meta-analysis found that the proportion of asymptomatic infections among COVID-19 positive persons varied widely, with estimates ranging from 1.4% to 78.3% across different studies6. Another meta-analysis reported that approximately 15.6% of confirmed COVID-19 patients were asymptomatic at the time of detection, with nearly half of these patients developing symptoms later5.

In specific populations, the prevalence of asymptomatic cases can be even higher. For instance, among obstetric patients, 95% were asymptomatic at the time of testing, although 59% remained asymptomatic through follow-up3. Similarly, a study focusing on children found that 27.7% of COVID-19 positive children were asymptomatic, a higher proportion compared to the general population5.

Clinical and Radiological Characteristics

Despite the absence of symptoms, many asymptomatic COVID-19 patients exhibit abnormalities in clinical and radiological assessments. For example, a study found that 62.2% of asymptomatic patients had abnormal CT scans, with ground-glass opacities being the most common finding1. Another study reported that 41.7% of asymptomatic patients had bilateral lung involvement on CT scans5. These findings suggest that asymptomatic individuals can still have significant underlying pathology.

Transmission Potential

Asymptomatic individuals can still transmit the virus to others, making them a critical factor in the spread of COVID-19. A study highlighted that 18.8% of close contacts exposed to asymptomatic index patients tested positive for COVID-193. This underscores the importance of identifying and isolating asymptomatic carriers to prevent further transmission.

Implications for Public Health

The high prevalence and transmission potential of asymptomatic COVID-19 infections pose significant challenges for public health. Effective strategies must include widespread testing, including among asymptomatic individuals, to identify and isolate carriers. Additionally, contact tracing and continuous monitoring are essential to control the spread of the virus.

 

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Is it common to have COVID-19 without getting symptoms?

Ivo Mueller has answered Likely

An expert from Walter+Eliza Hall Institute of Medical Research in Immunology, Epidemiology

Two key issues in the epidemiology of COVID-19 that we still understand poorly is how common are asymptomatic infections and how much do these contribute to transmission?

The answers to these are not only essential for getting a better picture of how many people have already been exposed to SARS-CoV-2, but will also greatly affect our predictions on how the COVID-19 pandemic may develop over the coming months, and what interventions are most important to prevent second wave of cases and deaths.

This study on the Antarctic cruise ship passengers makes an important contribution to answer the first questions. Among the 128 passengers that contracted SARS-CoV-2 on board, 81 percent did not have any symptoms. In other words, there were 4 asymptomatic carriers for every ill passenger. If the same pattern is repeated elsewhere, this means that in countries that only test symptomatic cases, the true burden of infections may be 5 times higher than currently reported. We learn a lot less on the second question. On one hand, the high attack rate (59 percent SARS-CoV-2 positive) among passengers despite extensive containment measure, again indicates how highly infectious SARS-CoV-2 is in a confined environment like a cruise ship. On the other hand, there were some instances where only one passenger in a cabin did get infected. While this may be due to false negative test results, it is also possible that similar to increasing evidence in largely asymptomatic children, some adults with very low, asymptomatic infections may also be less infectious.

Determining the true infectiousness of asymptomatic carriers of all age must now be an urgent priority. This will require detailed contact tracing studies using both molecular and serological diagnostic including both clinical cases and asymptomatic carriers.

This answer was adapted from the original quote published at the Australian Science Media Centre.

 

Is it common to have COVID-19 without getting symptoms?

William Petri has answered Likely

An expert from University of Virginia in Immunology, Parasitology, Virology

In general, having an infection without any symptoms is common. Perhaps the most infamous example was Typhoid Mary, who spread typhoid fever to other people without having any symptoms herself in the early 1900s.

My colleagues and I have found that many infections are fought off by the body without the person even knowing it. For example, when we carefully followed children for infection by the parasite Cryptosporidia, one of the major causes of diarrhea, almost half of those with infections showed no symptoms at all.

In the case of the flu, estimates are that anywhere from 5% to 25% of infections occur with no symptoms.

For the most part, symptoms are actually a side effect of fighting off an infection. It takes a little time for the immune system to rally that defense, so some cases are more aptly considered presymptomatic rather than asymptomatic.

How can someone spread coronavirus if they aren’t coughing and sneezing?

Everyone is on guard against the droplets that spray out from a coronavirus patient’s cough or sneeze. They’re a big reason public health officials have suggested everyone should wear masks.

But the virus also spreads through normal exhalations that can carry tiny droplets containing the virus. A regular breath may spread the virus several feet or more.

Spread could also come from fomites – surfaces, such as a doorknob or a grocery cart handle, that are contaminated with the coronavirus by an infected person’s touch.

What’s known about how contagious an asymptomatic person might be?

No matter what, if you’ve been exposed to someone with COVID-19, you should self-quarantine for the entire 14-day incubation period. Even if you feel fine, you’re still at risk of spreading the coronavirus to others.

Most recently it has been shown that high levels of the virus are present in respiratory secretions during the “presymptomatic” period that can last days to more than a week prior to the fever and cough characteristic of COVID-19. This ability of the virus to be transmitted by people without symptoms is a major reason for the pandemic.

After an asymptomatic infection, would someone still have antibodies against SARS-CoV-2 in their blood?

Most people are developing antibodies after recovery from COVID-19, likely even those without symptoms. It is a reasonable assumption, from what scientists know about other coronaviruses, that those antibodies will offer some measure of protection from reinfection. But nothing is known for sure yet.

Recent serosurveys in New York City that check people’s blood for antibodies against SARS-CoV-2 indicate that as many as one in five residents may have been previously infected with COVID-19. Their immune systems had fought off the coronavirus, whether they’d known they were infected or not – and many apparently didn’t.

How widespread is asymptomatic COVID-19 infection?

No one knows for sure, and for the moment lots of the evidence is anecdotal.

For a small example, consider the nursing home in Washington where many residents became infected. Twenty-three tested positive. Ten of them were already sick. Ten more eventually developed symptoms. But three people who tested positive never came down with the illness.

When doctors tested 397 people staying at a homeless shelter in Boston, 36% came up positive for COVID-19 – and none of them had complained of any symptoms.

In the case of Japanese citizens evacuated from Wuhan, China and tested for COVID-19, fully 30% of those infected were aymptomatic.

An Italian pre-print study that has not yet been peer-reviewed found that 43% of people who tested positive for COVID-19 showed no symptoms. Of concern: The researchers found no difference in how potentially contagious those with and without symptoms were, based on how much of the virus the test found in individuals’ samples.

The antibody serosurveys getting underway in different parts of the country add further evidence that a good number – possibly anywhere from around 10% to 40% – of those infected might not experience symptoms.

Asymptomatic SARS-CoV-2 infection appears to be common – and will continue to complicate efforts to get the pandemic under control.

I have adapted this answer from my original article in The Conversation

 

Is it common to have COVID-19 without getting symptoms?

Monica Gandhi has answered Likely

An expert from University of California, San Francisco in Infectious diseases

Yes, it is very common. It depends on the setting (young people or not, masking or not) but current best estimates place the rate of asymptomatic infection with SARS-CoV-2 at 40%.

What does it mean to be asymptomatic?

SARS-CoV-2 – the virus that causes COVID-19 – can produce a range of clinical manifestations.

Some people who are infected never develop any symptoms at all. These patients are considered true asymptomatic cases.

When people do get sick from the coronavirus, it takes on average five days and as many as two weeks to develop symptoms that can range from very mild to extremely dangerous. The time between initial infection and the first symptoms is called the pre-symptomatic phase.

As an infectious disease physician, when I hear about asymptomatic spread of SARS-CoV-2, I think of a person who doesn’t have symptoms at the moment they give the virus to someone else. It doesn’t matter whether they are a true asymptomatic case or just pre-symptomatic; the public health risk is the same.

How many people are asymptomatic?

Estimates of the proportion of true asymptomatic cases – those who are infected and never develop symptoms – range from 18% to over 80%. The reasons for the huge range in estimates are still unclear, but some studies are better than others.

The most accurate way to determine the rate of asymptomatic cases is to test people regardless of whether or not they have symptoms – an approach called universal mass testing – and track them over time to see if they develop symptoms later. A recent mass testing campaign in San Francisco found that 53% of infected patients were asymptomatic when first tested and 42% stayed asymptomatic over the next two weeks.

Another recent paper compared the evidence from 16 studies and estimated the overall rate of asymptomatic infection to be 40%-45%. This is in line with the San Francisco finding, but the studies sampled were of various quality and size and likely include some pre-symptomatic cases.

Though none of these studies is perfect, a lot of evidence supports a true asymptomatic rate of around 40%, plus some addition fraction of patients who are pre-symptomatic.

How can asymptomatic people spread the coronavirus?

Compared to most other viral infections, SARS-CoV-2 produces an unusually high level of viral particles in the upper respiratory tract – specifically the nose and mouth. When those viral particles escape into the environment, that is called viral shedding.

Researchers have found that pre-symptomatic people shed the virus at an extremely high rate, similar to the seasonal flu. But people with the flu don’t normally shed virus until they have symptoms.

The location of the shedding is also important. SARS-CoV – the virus that caused the SARS epidemic in 2003 – does not shed very much from the nose and mouth. It replicates deep in the lungs. Since SARS-CoV-2 is present in high numbers in a person’s nose and mouth, it is that much easier for the virus to escape into the environment.

When people cough or talk, they spray droplets of saliva and mucus into the air. Since SARS-CoV-2 sheds so heavily in the nose and mouth, these droplets are likely how people without symptoms are spreading the virus.

How much asymptomatic spread is happening?

Public health experts don’t know exactly how much spread is caused by asymptomatic or pre-symptomatic patients. But there are some telling hints that it is a major driver of this pandemic.

An early modeling estimate suggested that 80% of infections could be attributed to spread from undocumented cases. Presumably the undocumented patients were asymptomatic or had only extremely mild symptoms. Though interesting, the researchers made a lot of assumptions in that model so it is hard to judge the accuracy of that prediction.

A study looking at outbreaks in Ningbo, China, found that people without symptoms spread the virus as easily as those with symptoms. If half of all infected people are without symptoms at any point in time, and those people can transmit SARS-CoV-2 as easily as symptomatic patients, it is safe to assume a huge percentage of spread comes from people without symptoms.

Even without knowing the exact numbers, the Centers for Disease Control and Prevention believes that transmission from people without symptoms is a major contributor to the rapid spread of SARS-CoV-2 around the world.

What can we do to limit asymptomatic spread?

Any time a virus can be spread by people without symptoms, you have to turn to preventative measures.

Social distancing measures and lockdowns work, but have large economic and social repercussions. These were necessary when epidemiologists didn’t know how the virus was spreading, but now we know it sheds at high quantities from the upper respiratory tract.

This means that universal mask wearing is best tool to limit transmission, and there is evidence to back that idea up.

On April 3, the CDC recommended that all members of the public wear facial coverings when outside of the house and around others. The World Health Organization finally followed suit and recommended universal public masking on June 5.

At this point, no one knows exactly how many cases of COVID-19 are from asymptomatic spread. But I and many other infectious diseases researchers are convinced that it is playing a major role in this pandemic. Wearing a mask and practicing social distancing can prevent asymptomatic spread and help reduce the harm from this dangerous virus until we get a vaccine.

I have adapted part of this answer from my original article in The Conversation

 

Is it common to have COVID-19 without getting symptoms?

Sanjaya Senanayake has answered Likely

An expert from Australian National University in Infectious diseases

There have been varying proportions of asymptomatic cases in differing studies e.g. about 40% in study from Iceland, 18% on a cruise ship, 30.8% in Japanese evacuees from Wuhan and almost 80% in another Chinese study. So it’s hard to know which is right. And although we are getting closer to understanding the proportion of asymptomatic cases with COVID-19, we still don’t know for sure the magnitude of the impact that they have on further transmission of cases i.e. do they generate lots of secondary cases or only a small proportion?

 

Is it common to have COVID-19 without getting symptoms?

Isaac Bogoch, MD has answered Likely

An expert from University Health Network in Infectious diseases, Clinical Research, Epidemiology

A great study here on asymptomatic COVID-19 infection & transmission where 14000 tested via PCR, 49 positive, 30 participated in study. 13/30 (43%) NEVER had symptoms (truly asymptomatic).

Asymptomatic have lower viral load & faster time to viral clearance, “asymptomatic participants had a lower probability of having a positive RT-PCR result (i.e. a faster viral clearance) than symptomatic participants”. Of 30 asymptomatic cases: “Two of the asymptomatic individuals appeared to transmit the infection to up to four contacts.”

Take away points:

-This is a careful study that really defined those with no symptoms

-Those without symptoms probably less likely to transmit virus

-Due to high proportion of asymptomatic individuals, they still contribute to driving pandemic.

 

Is it common to have COVID-19 without getting symptoms?

Ben Cowling has answered Unlikely

An expert from University of Hong Kong in Infectious diseases, Epidemiology

Some people will get infected with SARS-CoV-2, and clear the infection after a period of time, and not experience any symptoms throughout the infection. These are called asymptomatic infections.

Based on data I have seen on systematic PCR testing of persons exposed to infection, and in quarantine, I believe asymptomatic infections occur in a minority of infections, perhaps around 20%.

Serological testing may be able to identify additional asymptomatic infections that PCR has missed, so in future we may come up with an estimate that exceeds 20%.

How does this compare with the flu?

For comparison, in influenza around 15%-20% of infections identified by systematic PCR testing of exposed persons are asymptomatic, but asymptomatic fraction is much higher if using serology to identify infections.

Among persons infected with SARS-CoV-2, there is absolutely no doubt that those who have symptoms are more contagious, on average, than those who have no symptoms.

Do Asymptomatic cases spread the virus?

Coughing, sneezing, runny nose, etc. are all ways that help virus to get out into the environment and then into other peoples’ respiratory tracts. Coughs and sneezes spread diseases!

However, we need to distinguish /contagiousness/ from /transmissibility/. If symptomatic cases stay at home, as they are advised to do, they won’t have so much opportunity to spread. Asymptomatic cases could have more opportunities to transmit infection to others.

In this study, we estimated that 44% of onwards transmission in China occurred from cases before their symptoms appeared (“pre-symptomatic transmission”). This estimate should be interpreted within the context of aggressive case finding and isolation in China, which would reduce post-symptomatic transmission. Pre-symptomatic transmission might play a smaller role in other places where cases do not self-isolate.

Going back even further, in this study we reported (in Table 1) that at least half of the first 425 cases in China did not recall any contact with a person with respiratory symptoms. Sometimes symptoms may not be recognised or reported. 

Some “asymptomatic” individuals may actually have a mild fever which they have not noticed, others may not want to reveal that they have lost their sense of smell because then they can’t go to work.Nevertheless, this is still chalked up to asymptomatic/pre-symptomatic transmission, and cannot be stopped by policies that focus on people recognising and reporting symptoms.

It is clear that pre-symptomatic transmission can occur, and contact tracing should start a few days before illness onset, rather than on or after illness onset.

I think it is less clear whether truly asymptomatic cases play a major role in transmission, and I think it is fair for WHO to say at this stage that asymptomatic cases seem to play a minor role in spreading infection.

Takehome message

Physical distancing continues to be an important public health measure, and I think face masks should be worn in public transport and crowded areas where physical distancing is difficult because these can reduce transmission in the community.