How many SARS-CoV-2 infections are asymptomatic?

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A recent study found that 40.5% of people infected with SARS-CoV-2 do not develop symptoms. Andrew Aitchison / Getty Images
  • Researchers looked at the proportion of confirmed SARS-CoV-2 infections that are asymptomatic.
  • They found that in different settings, 40.5% of people who contract the virus have no symptoms.
  • Researchers say the high percentage of asymptomatic SARS-CoV-2 cases highlights a potential risk of transmission within communities.

As of December 16, 2021, there had been more than 272 million confirmed SARS-CoV-2 infections worldwide, as well as more than 5.3 million confirmed COVID-19 deaths.

Symptoms of COVID-19 vary in severity, with some people showing symptoms of a mild respiratory infection and others developing pneumonia or acute respiratory distress syndrome. Some people, however, do not develop any symptoms and are asymptomatic.

Those who are asymptomatic may not seek medical attention, and the temperature screening may not detect that they have the infection. However, the virus can still transmit asymptomatic people.

One to study found that upper respiratory tract viral loads in asymptomatic people were similar to those in symptomatic people. Another one to study suggested that infectivity peaks at or before the onset of symptoms.

Unlike other viruses, such as the virus that causes severe acute respiratory syndrome (SARS), there has been little comprehensive research on transmission rates among asymptomatic people infected with SARS-CoV-2. Knowing these rates could help improve public health policies to manage the virus.

In a recent study, researchers at Peking University in China reviewed 95 studies, involving a total of nearly 30 million people. They calculated the percentage of asymptomatic people among those with confirmed infections, as well as among those undergoing tests.

They found that of all those tested, 0.25% had SARS-CoV-2 infection and were asymptomatic. Of those who received a positive test result, 40.5% were asymptomatic.

“The high percentage of asymptomatic infections in this study highlights the potential risk of transmission of asymptomatic infections in communities,” write the authors of the article.

The study appears in JAMA network open.

The researchers included 95 studies of various types written in Chinese and English that contained data on the health of 29,776,306 people. Among these studies:

  • 35 took place in Europe
  • 32 were in North America
  • 25 were in Asia

Of all the people who tested for SARS-CoV-2, the researchers found that 11,516, or 0.25%, had an infection but no symptoms. This figure varied, however, depending on the context. For example, asymptomatic cases were present among:

  • 4.52% of residents or nursing home staff tested
  • 2.02% of travelers tested on planes or cruises
  • 2.34% of pregnant women tested
  • 1.46% of close contacts tested
  • 0.75% of healthcare workers tested or hospitalized patients
  • 0.40% of community residents tested

The researchers further found that the percentage of asymptomatic cases was higher:

  • in developed countries
  • among populations with an average age of 60 years or older
  • in studies with a sample size of less than 100

When analyzing data for the percentage of asymptomatic cases among the population confirmed to have the infection, the researchers excluded 18 studies because all of their confirmed cases were asymptomatic.

The remaining 77 studies included 19,884 confirmed SARS-CoV-2 infections, of which 11,069 were asymptomatic.

The researchers found that among those infected with SARS-CoV-2, the cumulative percentage of asymptomatic infections was 40.5%.

Among the infections confirmed by SARS-CoV-2, the percentage of asymptomatic infections varied depending on the setting:

  • 54.11% in pregnant women
  • 52.91% of air and cruise travelers
  • 47.53% among residents or nursing home staff
  • 39.74% among community residents
  • 30.01% among healthcare workers and inpatients
  • 26.94% in close contact

These rates also varied by continent. While they accounted for 46.32% of confirmed infections in North America, they accounted for 44.18% in Europe and 27.58% in Asia.

The researchers also found that the rate was higher among those under the age of 20.

To explain their findings, the researchers say the lower rates of asymptomatic cases in Asia could be due to the large SARS-CoV-2 screening program in China.

They add that the percentage of asymptomatic cases was higher when excluding studies with large sample sizes.

The researchers further explain that people under the age of 39 were more likely to have asymptomatic infections than those in the older age groups. This, they say, is likely due to the fact that young adults are more likely to show only mild or moderate clinical symptoms.

“These results suggest that asymptomatic infections may contribute to the transmission of SARS-CoV-2 within the community,” the researchers write.

“To prevent further transmission in communities, asymptomatic individuals in the general population should be tested. If resources are limited, workers in specific industries, such as airline transport, should be tested regularly. “

“In addition, we found that about a third of people with [SARS-CoV-2 infections] were asymptomatic in healthcare workers or hospitalized patients. Since asymptomatic healthcare workers can contribute to the spread of disease inside and outside hospitals, surveillance of asymptomatic people is important for infection control and reduction of transmission in facilities. health and [the] community, ”they add.

The researchers conclude that the high percentage of asymptomatic infections highlights a potential risk of transmission within communities.

The authors explain that their study has several limitations. First, their research did not include pre-print studies, so they may have missed some relevant data.

The researchers also did not search Chinese literature databases, which means they may have missed some relevant articles written in Chinese.

They also explain that most studies did not include follow-up to identify presymptomatic and secret cases of COVID-19.

Medical News Today speak with Prof Irene Petersen, professor of epidemiology and health informatics at University College London in the UK, who was not involved in the study. She said:

“[This study] is a mixture of many different study designs and, therefore, an overall estimate of the proportion of asymptomatic cases should be viewed with some uncertainty. “

“Additionally, we need to be aware that the symptoms of COVID-19 have changed over time. So many would consider symptoms such as headaches and runny nose to be symptoms of COVID-19 as well in addition to the three classic symptoms, ”she added.

However, when we asked if this study was of good quality, Prof. Petersen confirmed: “Yes. This is a good study that seeks to summarize our current knowledge about COVID-19 test results. “

“The results are not surprising, as we have known for a long time that many people can get COVID without having the ‘typical’ symptoms of COVID-19. Some people may develop symptoms several days after [their infection becomes transmissible], “she added.

“While [the virus may be more likely to spread from] people with symptoms, they will often isolate themselves because they are aware that [they have the infection]. [Meanwhile], those who have no symptoms or those who are presymptomatic will not know they are [have the infection] and therefore, they may present a greater risk of transmission.


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