Since the beginning of the pandemic, there have been cases where infected people with SARS-CoV-2 did not experience any symptoms throughout the disease. Most often, such cases were detected by accident, since patients with symptoms were prioritized in PCR testing. The number of people with an asymptomatic form and their degree of infection was unknown, which adds uncertainty to the picture of the pandemic.

How many people don’t experience any symptoms with COVID-19?

The first published scientific data were frightening: 81% of cases were asymptomatic. The British medical journal published the results of PCR testing on COVID-19 of all passengers and crew members of an isolated cruise ship. A total of 217 people were tested, of which 128 (59%) were infected with the coronavirus. Moreover, 104 (81%) of the infected people had no symptoms of the disease. After this publication, some researchers suggested that asymptomatic carriers of coronavirus infection are the “silent driving force” of the pandemic.

Systematic review and meta-analysis: estimating the number of asymptomatic COVID-19

Certainty in estimating the number of asymptomatic COVID-19 cases came after the publication of a systematic review and meta-analysis in October 2020. Scientists at the Australian Institute of evidence-based medicine sought to summarize all available research on asymptomatic cases and the transmission rate of SARS-CoV-2.

The researchers selected 2,454 articles and 13 studies that reported asymptomatic spread. All publications had a low risk of systematic error:

  • The structure of the sample of people included the risk group.
  • Participants were monitored for at least 7 days.

In total, these publications show the results of the RT-PCR test of 21,708 people. The fact of the disease was confirmed in 663 cases, and 111 – in an asymptomatic form. The percentage of asymptomatic instances was in the range of 4% – 41%. The meta-analysis showed that the overall rate of asymptomatic patients was 17%.

Separately, the researchers stressed that more data is needed on the proportion of asymptomatic coronavirus in children to understand better the importance of symptom-free behavior in accelerating the spread of the pandemic.

How contagious are people with COVID-19 who don’t have a single key symptom?

A meta-analysis conducted by scientists from the Australian Institute of evidence-based medicine shows that the risk of COVID-19 transmission in the asymptomatic period is 42% lower than in the presence of symptoms.

The researchers note that five studies provided evidence of direct transmission of coronavirus infection from an asymptomatic carrier to a healthy person.

The role of an asymptomatic form of coronavirus infection in the spread of the disease

A large population study conducted in Switzerland is devoted to the study of the role of asymptomatic carriers in the spread of coronavirus infection. Scientists from the University of Geneva (Switzerland), together with experts from Johns Hopkins University (USA), conducted a serological survey of 4,524 people aged 5 years and older from 2,267 households.

Scientists have suggested that serological testing for IgG antibodies to SARS-CoV-2 is more likely to cover all previously infected people than virological PCR testing, which skips mild and asymptomatic cases.

Purpose of the research:

  • Assess the risk of transmission of asymptomatic infections at home
  • To identify the critical risk factors for transmission and infection with SARS-CoV-2

The researchers stressed that knowledge about the possibility of transmission of asymptomatic infections and the risk of disease at home is crucial for the fight against COVID-19.

As a result of the serological examination and subsequent modeling, it was found that:

  • Overall, the risk of infection from a single-family member infected with coronavirus is 17.2%. Moreover, the risk increases with age: among children aged 5-9 years, the risk is 7.5%, and in the elderly 65+, the risk is 30.2%. The risk also increases with increasing household size: from 16.1% for a household of 2 people to 41.2% for a household of 5 people.
  • The risk of infection from an asymptomatic seropositive household member is 4 times lower (74.8%) than with symptoms.
  • The absolute risk of infection outside the household is 5.1%.
  • IgG antibodies to SARS-CoV-2 were detected in 6.6% of people.
  • 9.8 % of households had at least one family member with antibodies.

For more information about the timing of antibody production and the duration of protective immunity, see the article “Protective immunity after COVID-19 in children and adults”.

Conclusions

About one in five people infected with the coronavirus does not experience symptoms. The risk of infection from an asymptomatic carrier is 2 to 4 times lower than from a person with symptoms.

The probability of infection at home from an infected family member is about 3 times higher than outside the household. The risk of disease increases with age.

People without symptoms should follow prevention measures, as there is direct evidence of transmission from an asymptomatic carrier of COVID-19 to a healthy person. For more information about prevention measures, see the article “List of measures to reduce the risk of COVID-19 infection”.

Sources

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