Corona possible without symptoms?

Is the corona virus contagious even without symptoms?

It is still unclear how many people have been infected with the SARS-CoV-2 coronavirus. The proportion of illnesses that plays without or with very mild symptoms plays an important role here. The first studies now provide clues for this proportion.

The people who tested positive for SARS-CoV-2 already show that around 80 percent of all COVID-19 diseases are mild or moderate. These cases can be treated in home quarantine. However, it is unclear how many people have infected themselves unknowingly and unnoticed.

Why is the real figure so important?

The number of laboratory-confirmed coronavirus cases is only of limited significance. If you want to know how quickly the pandemic can be defeated, you need to know the undisclosed number. This is the only way to determine how widespread the virus is and what immunities prevail in the population against SARS-CoV-2. As a rule of thumb, if around 70 percent of a population has immunities, the epidemic is considered defeated.

Corona: How can the real figure be determined?

The number of unreported cases can be determined by not only testing people with symptoms or people who have been in contact with infected people, but also random samples of the population - as many as possible. However, due to the limited test capacities, this has so far been difficult to carry out. Some studies have done this in small isolated populations.

The village with the first Corona death in Italy

For example, researchers from Imperial College London conducted a study in a small Italian village where almost all of the residents were tested to determine the undisclosed number. The study results can be viewed on the preprint server "medRxiv".

3,304 people live in the northeastern Italian community of Vo. On February 21, 2020, a resident of the municipality died of pneumonia due to a SARS-CoV-2 infection. This was the first COVID-19 death to be found in Italy. In response, the regional authorities blocked the entire community for 14 days.

Almost an entire community has been tested

A Imperial College research team led by Neil Ferguson of London, whose team is one of the world's leading epidemiologist groups, tested a large part of the population in the small cordoned-off area at the beginning and end of the contact block to find out more about the number of unreported cases, to maintain herd immunity and the spread of coronaviruses.

Corona infections possible without symptoms

At the beginning of the isolation, the researchers tested 85.9 percent of the population in Vo and at the end of the isolation another 71.5 percent of all people living there. Through extensive testing, the team found that 2.6 percent of the community was already infected at the start of isolation. At the end of the contact block, only 1.2 percent of the residents were infected. Through surveys of the infected people, it was found that 43.2 percent of all illnesses were asymptomatic, ie without complaints.

Corona infectious without symptoms

By tracking infection chains, the research team was also able to show that asymptomatic people were responsible for the majority of new infections that occurred in households during isolation. The study is one of the first pieces of research to shed light on the frequency of asymptomatic SARS-CoV-2 infections and provides new insights into transmission dynamics and the effectiveness of measures such as isolation and contact restrictions.

Drosten is impressed with the study

Even if the study has not yet appeared in a specialist journal, the results have given the virologist Professor Dr. Christian Drosten impressed. According to the coronavirus expert, the 43.2 percent asymptomatic cases were determined by precise data collection. "I think that's a number you can work with in the future," Drosten said in his NDR podcast.

The Heinsberg study in Gangelt

A similar research project was carried out by the state government in North Rhine-Westphalia under the leadership of the renowned virologist Professor Dr. Hendrik Streeck in Gangelt, Heinsberg district. However, since the population in Gangelt is much larger, it was only tested on a random basis.

The research project “Covid-19 Case Cluster Study” came to the preliminary conclusion that at the time of the study (early April 2020) around two percent of the population in Gangelt had an acute COVID-19 disease. Around 14 percent of the population sample showed antibodies against SARS-Cov-2, which indicates immunity. In total, around 15 percent of the residents of Gangelt were infected with the novel corona virus.

Corona: What is the death rate?

From this result, it could also be derived for the Heinsberg region that the corona death rate is significantly lower than previously assumed. As a result, 0.37 percent of those infected died. This rate is well below the 1.98 percent lethality reported by Johns Hopkins University. The Heinsberg study clearly shows that mortality can be depicted more realistically if the number of asymptomatic cases is taken into account.

In this context, virologist Streeck also emphasizes that it can be expected that strict hygiene measures and the isolation measures taken will keep the death rate low. Exceptional outbreaks ("superspreading events") such as carnivals, parties, discos, concerts, soccer games, the Oktoberfest and the like could result in uncontrolled spreads. This could undermine the health care system and cause death rates to skyrocket, similar to Spain, Italy and the United States.

Antibody tests should reveal the undisclosed figure

The problem with the studies currently available, however, is that they are isolated and relatively small communities. What is valid in Vo or Gangelt, for example, does not necessarily have to be transferable to large cities such as Berlin and Munich. The Robert Koch Institute (RKI) now wants to reveal the number of unreported cases at federal level. Several nationwide antibody studies are planned for this.

Corona: How is herd immunity determined?

In the studies, some of which have already started, 5,000 blood samples for SARS-CoV-2 antibodies are to be tested nationwide every 14 days. In addition, the RKI plans to examine 2,000 participants from particularly affected locations in Germany at regular intervals. Clinical symptoms, previous illnesses, health behavior, living conditions and mental health should be taken into account. The first results from these studies are expected in early May 2020.

In addition, the RKI plans to conduct 15,000 random samples at 150 locations in Germany from mid-May to measure the prevalence of antibodies in the population. These results could be of great importance in order to be able to assess the course and severity of the pandemic more precisely. The first results are expected in June 2020. (vb)

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Graduate editor (FH) Volker Blasek


  • Enrico Lavezzo, Elisa Franchin, Constanze Ciavarella, u.a .: Suppression of COVID-19 outbreak in the municipality of Vo, Italy; in: medRxiv, 2020,
  • NDR: Coronavirus update (33) with Christian Drosten: Herd immunity is far from in sight (published: April 20, 2020),
  • Robert Koch Institute (RKI): How many people are immune to the new corona virus? Robert Koch Institute starts nationwide antibody studies (published April 9, 2020),
  • Prof. Dr. Hendrik Streeck, u.a .: Preliminary results and conclusions of the COVID-19 case cluster study (Gangelt municipality) (published: 09.04.2020),
  • NDR: Coronavirus update (29) with Christian Drosten: Use tests more specifically (published: April 8th, 2020),

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