Covid-19 or the flu? New antigen tests are capable of differentiating between the two

As the autumn-winter season approaches and the influenza virus has regained seasonality, new diagnostic kits have the ability to detect both diseases at the same time

Covid-19 or the flu? Although the symptoms of both viruses are practically indistinguishable from each other, as of this autumn it will be possible to tell the difference. For the first time since the coronavirus pandemic swept across the globe in early 2020, pharmacies are now stocked with tests that are capable of detecting both Covid-19 and influenza. These antigen tests are almost identical to the ones that have become so familiar during the pandemic, but now they are also able to detect the flu virus separately.

The autumn-winter season in 2022 in the Northern Hemisphere will arrive with both pathogens hand in hand, something has not yet occurred since the beginning of the pandemic. It has already happened in the Southern Hemisphere, where influenza has returned to a seasonality – albeit somewhat earlier than would usually be the case – that it had temporarily lost due to the interference of Covid-19 and the measures put in place to curb its spread.

In Spain – and it can be extrapolated therefore in Europe in general – the latest data suggests something similar will happen. The epidemiological bulletins produced by the Ministry of Health place the incidence rate of both pathogens at a practically equal level. Infections have been on the rise, moderately but steadily, for a little over three weeks.

The procedure for the combined antigen tests is the same as that for Covid-19 tests: a sample is taken from either the nose or mouth, depending on the type of test purchased, using the swab provided and then mixed with the solution included in the diagnostic kit. From there, there are two different kinds of test kit, one with two small receptacles in which to place the sample – one for Covid-19 and one for flu – and another that has just one. In both cases, the red line determines whether coronavirus or flu (types A and B) antigens have been detected.

The active cycle of both viruses has a similar duration: they take between one and four days to incubate, and the infection generally lasts for between eight and 10 days. María del Mar Tomás of the Spanish Society of Infectious Diseases and Clinical Microbiology notes that antigen tests are very reliable in people who return a positive result, but not so much when the result is negative. “There could have been an error when collecting the sample, the virus may still be in the incubation period or the viral load could be insufficient,” she says.

As such, the expert recommends that people who are displaying symptoms compatible with both diseases should observe basic precautions so as not to risk infecting other people, particularly the elderly and people with immune system problems, who are most at risk of being hospitalized or dying from Covid-19 or the flu.

As things stand, there is nothing to suggest that either Covid-19 or the flu will be more severe on this occasion than during previous waves, during which fatality and hospitalization rates have been much lower than during the early stages of the pandemic. If the Omicron variant continues to behave in the same way as it has up to now, it is foreseeable that transmission rates will be high, but that there will not be as significant an impact on public healthcare systems as in 2020 and 2021.

At the moment, the predominant strain is the same one that caused the seventh Covid-19 wave: BA.5, a subvariant of Omicron, although others have been detected that could replace it. Studies published so far refer back to the original Omicron strain; one from July shows that antigen tests were still returning positive results in the majority of infected people (83%) five days after the first symptoms emerged. Over time, that number drops. In a majority of cases, the infection disappears after eight to 10 days, but 13% of people still remained positive after that timeline had elapsed. In general, a positive test result is correlated to the ability to infect other people, something that should be taken into account when taking a test.

Another study published in October examined the most frequent symptoms among a group of 3,000 people who tested positive for Omicron. These were: coughs (67%), a sore throat (43%), nasal congestion (39%), and headaches (35%). The least recorded were loss of smell (5%) and diarrhea (5%).

The new tests make it possible to determine if these symptoms are being caused by Covid-19 or flu. Beyond the curiosity factor, this could prove to be important for older patients or those with immune deficiencies, who can proceed with one course of treatment or another depending on the illness, says Roger Paredes, head of the infectious diseases department at the Hospital Trias i Pujol in Badalona, Catalonia. For the rest of the population, precautions to avoid infecting other people should be the same in both instances, particularly in the case of the most vulnerable.

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