COVID-19, Cold and Flu: How to Distinguish Them?

COVID-19, Cold and Flu: How to Distinguish Them?

Influenza and respiratory syncytial virus epidemics have (almost) disappeared since the onset of the COVID-19 pandemic. Will it be the case in the 2022-2023 cold season? Or should we prepare for the coexistence of Covid-19 and influenza epidemics?

In the Boreal Hemisphere autumn has just begun, bringing with it the lowering of temperatures which, together with factors such as the reopening of schools and the massive resumption of work activities, the fact of being more indoors, temperature changes, favors the spread of flu.


Description of the virus and related disease (1,2,3)

Influenza is a disease caused by the infection of the respiratory tract by the flu virus. There are currently two types of viruses: A and B. Influenza Type A virus is classified into subtypes based on the surface proteins that characterize them. In particular, the subtypes HA (from hemagglutinin protein) and NA (from neuraminidase protein) are distinguished. These subtypes are further differentiated, with subtypes H1 and H3, and subtypes N1 and N2 being the most frequent outbreaks that have occurred in recent decades. Influenza Type B virus is divided into B/Yamagata /16/88 and B/ Victoria/2/87-like viruses. While influenza A viruses are prevalent in annual epidemics, influenza B viruses contribute to the epidemic in a variable manner over the years. In Europe, the most widespread strain in recent years is influenza A H3N2.

Like any other virus, the influenza virus continuously undergoes genome changes too, generating antigenically different strains (a phenomenon known as “antigenic drift“). This is the reason why flu vaccines need to be updated every year to adapt them to the strain that is widespread in the season. Influenza is one of the few viral diseases that an individual can contract repeatedly over the course of his life, regardless of age, lifestyle and where he lives.

The most frequent symptoms of the flu are the sudden rise in fever and the onset of cough and muscle aches. Other common symptoms include sore throat, headache, chills, fatigue, and loss of appetite. In children, nausea, vomiting, and diarrhea may also occur. The course of the disease for most people is between a week to ten days, while those at risk may experience more serious complications or a worsening of their underlying condition.


Spread and Transmission (4,5,6)  

The influenza virus spreads mainly in the winter season, although there are sporadic cases of infection at other times of the year. Young children and the elderly are most at risk of developing diseases such as viral pneumonia, and secondary bacterial pneumonia. Out of age, even all those who are considered “fragile” subjects because they are carriers of predisposed diseases are particularly at risk of complications of previous conditions in case of severe influenza infection. Other subjects at risk are patients with co-morbidities, patients hospitalized or housed in social and healthcare facilities, and pregnant women.

Influenza is mainly transmitted by droplets spread through coughing or sneezing and through direct or indirect contact with contaminated respiratory secretions. The incubation period varies from one to four days. Adults may be able to spread the flu to others from one day before symptoms to about five days after symptoms. Children and people with weakened immune systems can be more contagious. Healthcare professionals and staff employed in healthcare facilities play an important role in the spread of the virus among “fragile” patients.

It is estimated that, globally, there are about one billion cases of influenza a year, of which between 3 and 5 million turns into cases with serious complications, and between 290,000 and 650,000 deaths. In Europe, ECDC, the European Disease Control Center, estimates symptomatic cases of influenza in the range of 4 to 50 million, with 15-17,000 deaths from consequences associated with the infection. 90% of deaths occur in adults over the age of 65 with chronic clinical conditions (pre-COVID-19 data).

According to an estimate by the WHO, a flu epidemic costs about 60 million dollars every year, which could be reduced to 4.5 million by adopting the appropriate prevention measures.

In fact, in the last two winters (seasons 2019-2020 and 2020-2021), thanks to the measures to contain and prevent the spread of the SARS-CoV-2 virus, the incidence of influenza has been negligible. This implies also that the population, having been less exposed to the virus, could have a weakened immunity against influenza, especially in children and young people. The fear of experts is therefore that there will be significant outbreaks, especially if new or recently introduced strains emerge.


Prevention and diagnosis (7,8)

The WHO, World Health Organization, recommends annual vaccination to reduce the spread of influenza and the risk of serious effects in the event of infection.

In addition, good prevention practices are avoiding contact with infected people, avoiding touching hands, mouth, and eyes, washing hands regularly, and disinfecting the surfaces in rooms shared with infected people.

Influenza is diagnosed on the basis of clinical evaluation which can be supported by rapid antigen tests or molecular RT-PCR tests.



Description of virus and related disease (9, 10, 11)

Respiratory syncytial virus (RSV) is a single-stranded RNA virus made up of 10 genes encoding 11 proteins. The scientific name is Human orthopneumovirus and belongs to the Orthopneumovirus genus, Pneumoviridae family. Based on the antigenic reactivity of proteins F and G to monoclonal antibodies, RSV is distinguished into subunits A (RSVA) and B (RSVB). Although the two subunits circulate among the population at the same time, RSVA is more common, as well as having a higher viral load and faster transmission time.

Worldwide, RSV is the leading cause of bronchiolitis and pneumonia in infants and children under 5 years of age, with 2-3% of cases requiring hospitalization.

The most common symptoms, which arise in stages, are runny nose, decreased appetite, cough, sneezing, fever, and wheezing. In very young children, the only symptoms may be irritability, reduced activity and difficulty breathing.


Spread and Transmission (10,12)

It is estimated that around 30 million acute respiratory infections and more than 60,000 infant deaths occur each year from RSV. Even for adults and the elderly with underlying heart and lung diseases, RSV infection can be dangerous for the patient’s health and life-threatening. Like the flu, RSV is also a seasonal virus.

However, during the Covid-19 pandemic, there was a significant reduction in the spread of RSV in cold seasons, followed by an off-season rebound of the disease once containment measures for SARS-CoV-2 were eased or removed.

As with other respiratory viruses, transmission occurs by contact with infected people or surfaces contaminated with breath droplets containing the virus. RSV enters the body through the eyes, mouth, and nose. People are usually contagious for 3 to 8 days, but children and “fragile” people can be symptomatic and remain contagious for up to 4 weeks.


Prevention and diagnosis (12,13)

Despite the high interest and efforts of the scientific community, there is still no vaccine available against RSV. Behaviors aimed at reducing the risk of contracting the virus, therefore, remain the most important means of prevention. In addition to washing your hands often, not touching your face, and avoiding contact with infected people, it is suggested to avoid prolonged stays in potentially contagious places and frequently clean games and surfaces accessible to children.

The diagnosis is based on the analysis of symptoms and can be supported by antigenic or molecular tests to confirm the presence of the virus in the body.



Description of the virus and the related disease  (14, 15, 16, 17, 18, 19, 20)

SARS-CoV-2, short for Coronavirus 2 from the severe acute respiratory syndrome, is a viral strain belonging to the subgenus Sarbecovirus, of the subfamily of coronaviruses (Orthocoronavirinae). SARS-CoV-2 is the seventh family virus known to infect humans. The SARS-CoV-2 first cases identified date back to the end of 2019, after the isolation of the virus from patients with very severe bilateral bronchitis, hence the name of the resulting disease, namely COVID-19. Analysis of the SARS-CoV-2 genome confirmed that the virus is present in bats, but it is not yet clear what was the mechanism that allowed the species to jump from bat to man (spillover). Like other coronaviruses, SARS-CoV-2 is made up of 4 structural proteins: protein S (spike or spinule), E (envelope), M (membrane), and N (nucleocapsid). The RNA genome is contained in the N protein while the other proteins together form the viral capsid. In particular, the Spike protein allows it to attach itself to the membrane of a host cell and promote infection. Precisely because of the importance of this ability to attack host cells, protein S is subjected to a selective pressure caused by immunized people, to which it responds with mutations in its gene that lead to variants of the virus. From the beginning of the pandemic to date, numerous variants have been identified, some of which have had a global spread, while others have remained confined to specific geographical areas. The Center for Disease Control and Prevention (CDC) in the US and the (ECDC) in Europe constantly monitor the epidemiology of infections and constantly update the list of variants to be monitored (VBM), variants of interest (VOI), and variants of concern (VOC). The latter are variants of viruses that constitute a global danger because they could lead to an increase in transmissibility, determine more severe diseases and a significant reduction in the protection of natural or vaccine immunizations, reduce the efficiency of treatments or vaccines and reduce the ability to be diagnosed. (

The most common symptoms of SARS-CoV-2 infection are fever, cough, fatigue, and loss of taste and smell. However, sore throats, headaches, muscle and skeletal pains, diarrhea, rash or discoloration of the fingers or toes, red or sore eyes may also occur. Severe symptoms are instead represented by difficulty in breathing or shortness of breath, loss of speech or mobility, confusion, and chest pains. Symptoms last on average 4 to 15 days. In severe cases, the disease evolves involving the failure of several organs until the patient dies.


Spread and transmission (21, 22)

According to data collected by the WHO, to date, there are more than 612 million confirmed cases of COVID-19 and more than 6.5 million deaths. The spread is worldwide: 41% of cases were registered in Europe, 29% in America, 14.5% in Asia Pacific, 9.8% in South East Asia, 3.7% in the eastern Mediterranean and 1, 5% in Africa. However, these data do not consider the number of tests actually carried out in the different regions, with a potential underestimation of real cases.

The SARS-CoV-2 virus enters the body through the nose, eyes, and mouth by contact with the hands contaminated by the droplets of breath released by the infected person.

The incubation period between infection and the onset of symptoms varies from 2 to 11 days, up to a maximum of 14 days. It is now known that different variants of the virus have different transmission, infection, and mortality capacities.


Prevention and diagnosis (23,24)

To reduce the spread of SARS-CoV-2 and the risk of serious effects in case of infection, it is important to complete the vaccination program, accordingly to the directions of the health system in your country.

Furthermore, as with influenza and other respiratory viruses, it is advisable to avoid contact with infected people, avoid touching hands, mouth and eyes, wash hands regularly and disinfect surfaces in rooms shared with infected people, keep a distance of at least one meter from other people and avoid crowded and unventilated places. The use of the mask, as a prevention device, is equally useful.

Although rapid tests are widely used in the population, in the case of infection, the only recognized diagnostic method for determining the presence of the virus in the body is the RT-PCR reaction made on a nasal or oropharyngeal sample or on saliva.

Most of the commercially available SARS-CoV-2 RT-PCR tests detect 2 or 3 genes of the virus; some of them detect the gene coding for the Spike protein (S-gene). It is important to note that the S gene is more subject to mutations than the other genes for the strategic function of anchoring to the host cell of the Spike protein, therefore the probability that a variant of the virus is not detected by the amplification of the S gene causing a false negative result is not to be underestimated. The first variant VOC B.1.1.7 was the case, for example.


WINTER SEASON 2022 – 2023: COVID-19, Influenza, RSV, and comorbidities (12, 25)

Already in the winter of 2021-2022, experts feared the simultaneous spread of the COVID-19 epidemic and flu or the phenomenon baptized as Twindemic. Luckily it did not occur mainly due to the restrictions introduced to marginalize COVID-19 infections in many countries. What will happen in the winter of 2022-2023? The WHO influenza observatory looks closely at the Australian flu season 2022 typical of the southern temperate regions (from June to September) as a potential predictor of what can happen in the northern hemisphere regions. In Australia, in the past few months, there has been a peak of flu infections higher than in the last 5 seasons with a higher incidence among children and adolescents than adults. Fortunately, the impact of incidence on hospital admissions was low to moderate. On the other hand, these data can be partially explained by the fact that for two years people have been less exposed to seasonal respiratory viruses and, therefore, the immunity of the population may have decreased compared to influenza and RSV. There could therefore be significant new outbreaks, especially if new or recent flu strains appear.

The effects of the Covid-19 infection peaks, the so-called “COVID-19 waves“, on national health systems, and the difficulties of reconciling the prevention and treatment of all other diseases with the management of Covid-19 patients are still alive in the memory of many. What are the solutions available to laboratories and hospitals to prevent, diagnose and manage a possible Twindemic? As described in the previous paragraphs, the symptoms between COVID-19 and Influenza are very similar and therefore it is difficult to distinguish the two diseases based only on symptoms, nonetheless, differentiation is of paramount importance in order to assess the most appropriate therapy since it differs depending on the type of infection. Fortunately, there are molecular tests that can detect the presence of SARS-CoV-2, Influenza, and RSV in the same sample, defined PCRs.



Sentinel Diagnostics has developed different diagnostic tests to detect SARS-CoV-2 alone or with other respiratory viruses. All kits are based on STAT-NAT®, Sentinel CH. S.p.A. proprietary formulation, highly stable at room temperature. STAT-NAT® kits are available for manual or for automated use, using SENTiNAT®200, Sentinel’s sample-to-results platform.  STAT-NAT® kits are easy to use, robust, and equipped with barcodes for the complete tracking of the assay workflow.

STAT-NAT® kits available are:

  • STAT-NAT® COVID-19 MULTI: RT-PCR kit for the qualitative detection of RdRP and Orf1b genes of SARS-CoV-2. Two formats for manual use are available: pre-filled 8-strips tube (96 reactions) and amber glass vial (288 reactions).
  • STAT-NAT® SARS-CoV-2: lyophilized Real-Time RTPCR multiplex assay for the qualitative detection of specific regions of SARSCoV-2 N gene and RdRP Gene and the E gene common to all Sarbecovirus, in human nasopharyngeal swab specimen. The kit allows fast and simple results evaluation. The use of highly conservative gene primers makes the kit more reliable to detect potential new variants. STAT-NAT® SARS-CoV-2 is intended for manual use in 2 formats: pre-filled 8-strips tube (96 reactions) and amber glass vial (288 reactions).
  • STAT-NAT® SN200 SARS-CoV-2 automated, lyophilized Real-Time RT-PCR multiplex for the qualitative detection of specific regions of SARSCoV-2 N gene and RdRP Gene and the E gene common to all Sarbecovirus, in human nasopharyngeal swab specimen. The kit is intended for use in an automated extraction and amplification system and it’s compatible with our SENTiNAT® sample-to-results system.
  • STAT-NAT®Pluri CoV-2/FLU/RSV is a manual lyophilized Real-Time RT-PCR multiplex assay, intended for the simultaneous qualitative detection and differentiation of RNA from the SARS-CoV-2, Influenza A&B (Flu A&B) viruses and Human Respiratory Syncytial Virus A&B (RSV) in human nasopharyngeal swabs collected in viral transport media. STAT-NAT®Pluri CoV-2-FLU-RSV is intended for use in a manual workflow, in 2 formats: pre-filled 8-strips tube (96 reactions) and vial (288 reactions).
  • STAT-NAT® SN200 Pluri CoV-2/FLU/RSV is an automated, lyophilized Real-Time RT-PCR multiplex assay based on nucleic acid amplification for the in vitro qualitative detection and differentiation of RNA from the SARS-CoV-2, Influenza A&B (Flu A&B) viruses and Respiratory syncytial viruses A&B (RSV A&B) in human nasopharyngeal swabs specimens. STAT-NAT® SN200 Pluri CoV-2/FLU/RSV is intended for use in an automated extraction and amplification system and it’s compatible with our SENTiNAT® sample-to-results system.


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