DR TANJA JOVANOVIĆ, VIROLOGIST, SPEAKS WITH KURIR: 'It is possible to have Covid and get multiple negative PCR test results.'

'If the sample is taken at the right time and from the right place, and is adequately processed, the result is reliable. If taken late, this may result in false negatives.'

When the throat or nose PCR test is negative, how often is a saliva specimen used to check for the coronavirus?

It is important for a patient to arrive in time, at the very start of the disease. If the sample is taken at the right time and from the right place, and is adequately transported and processed at the laboratory, then there is no mistake – the result cannot be a false negative. If, however, the disease has gone on for some time, and the virus is present in the lower respiratory tract, the throat and nose swab can be negative. In such cases it is better to examine the saliva specimen. Also, at the end of the disease, the throat and nose swab test results can be positive again as the virus is actively secreted from the body. This usually happens after two weeks.

What should be done when the PCR test result is negative, but Covid-19 is suspected?

The serological test is used as an additional test for this group of patients. This is a blood test which detects the presence of the IgM antibodies. Even if this test is negative, the clinical doctor assesses whether to treat such patients as Covid-19 patients based on the blood count and the laboratory and radiological findings.

So, it is all down to how good the doctor is?

It is always important for the doctor to monitor the patient.

How exactly does the coronavirus travel through the body?

The coronavirus is transmitted through droplets or an aerosol which is formed when an infected person coughs, sneezes, or talks. The virus remains infectious in droplets and aerosols for about two hours, depending on air temperature and humidity. The coronavirus receptors are located on the epithelial cells of the nose, pharynx, and the lung alveoli, where they are the most numerous, and the virus has a high affinity for binding to them. The virus actively replicates in the cells, producing a great many new particles. The outcome of the infection depends on the body's ability to fight it. If the virus is not eliminated from the respiratory tract, it goes on to attack the organs and organ systems which have the specific receptor for it.

What exactly does the virus do to the organs that it goes through?

There are two types of epithelial cells in the lung alveoli. The first type is involved in the gas exchange between the blood and the lungs, and the other in the production of the liquids which prevent the alveoli from collapsing. The coronavirus attacks just these cells and replicates inside them, which results in initiating a programmed death of the alveolar epithelial cells. So, the cells deteriorate, and lungs cannot work properly. The body then initiates the innate immune response, which is accompanied by the production of molecules recruiting white blood cells for the fight against the virus in the lungs. They also initiate the production of interferon, which has an antiviral effect. Most infected people manage to stop the further spread of the infection by secreting the virus fully into the environment through respiratory secretions. In people whose immune system is compromised, the virus can spread further and lead to a multi-system disease. The further replication of the virus in the body triggers a powerful immune response of the specialized lymphocytes. This in turn results in the production of excessive quantities of molecules which damage the infected organs in the fight against the virus. Especially important are infections of the heart, the kidneys, the gastrointestinal tract, and the central nervous system.

What does the clinical presentation of a person infected with Covid-19 depend on?

Most people will not have any significant symptoms of the disease. In epidemiological terms, these people are especially important as they spread the infection without knowing they are infected. Most infected people develop the clinical presentation of a respiratory infection. Pneumonia is the most frequent and often requires the patient to be hospitalized. Only a fraction of those infected develop multi-system diseases, and death occurs in three to five percent of symptomatic patients. Many host factors impacting on the severity of the clinical presentation have not been discovered yet. These factors are probably related to specific genes determining the quality of the immune response, or genes that the virus uses to successfully replicate in the body, e.g. the gene for a specific receptor that the virus exploits to infect cells. At the moment, what is known is that people with comorbidities, such as chronic non-infectious lung conditions and cardiovascular and metabolic diseases, as well as obese people and those older than 65, develop a more severe clinical presentation.

How and to what extent does the coronavirus mutate?

Coronaviruses do not mutate fast, as they have developed the capacity to correct errors which occur during replication. What we know now is that mutations occur in the entire genome, but mostly in the virus genes whose products are important for the interaction of the virus with the host. Research has shown that point mutations are the most frequent. At the moment, they do not have a big effect on the behaviour of the virus. Over time, however, as they accumulate, they can result in a significant change of virus virulence, in both directions (decreasing or increasing), which is relevant to the infection outcome. A mutated virus can still circulate in nature and cause new seasonal coronavirus infection epidemics.

Is there a specific coronavirus genome in Serbia?

European isolates are characterized by a specific mutation within the gene encoding the basic protein structure of the virus particle. This mutation is responsible for a faster transmission of the virus and is present in the virus isolates in Serbia.

(Kurir.rs/Politika)