Chief infectious disease specialist and head of the infection control department at West Tallinn Central Hospital, Pille Märtin, noted that each flu season has shown some distinct features since the COVID-19 pandemic. Compared with the past two seasons, this year's peak arrived somewhat later, she said.

"In the previous two seasons, flu began spreading in December, picked up pace in January and the flow of flu patients to the hospital gradually eased in the second half of February. This year, the peak has shifted to February and March and is only now showing signs of slowly declining," Märtin said.

She added that in earlier seasons — especially before the pandemic — A-type flu viruses tended to arrive at the beginning of winter, with B-type flu following in the spring. But this year, all types are circulating simultaneously. "Both A and B types are spreading: two strains of A-type flu and B-type flu," the chief physician confirmed.

Still, Märtin pointed out that the number of hospitalizations this season does not appear to be higher than in previous years. "In both the last and the season before that, around 1,300 people were hospitalized with the flu. About 10 percent of them required ventilator support or some other form of intensive care," she said. Compared with last year, doctors have observed a slight decline in hospitalizations among those over the age of 60.

"So far this season, just over a thousand people have been hospitalized with the flu, and again, about 10 percent have needed intensive care. However, the season isn't over yet, and we expect more hospitalizations in the coming weeks. At least based on our hospital's experience, someone is being admitted with flu-related pneumonia nearly every day," Märtin added.

Over the past three seasons, an average of 11 to 12 percent of Estonia's population has been vaccinated against the flu, Märtin pointed out. "That's actually ten times better than it was ten years ago. Back then, the coverage was somewhere around 2 percent," she noted. Vaccination rates have especially improved among the elderly and children.

In recent years, just over a quarter of older adults have gotten a flu shot. "So there has been progress. Last season, the Health Board reported that 14 percent of children were vaccinated. The season before, that figure was only 7 percent," said Pille Märtin.

Flu less severe but season longer

Kärt Sõber, head of the epidemiology department for infectious diseases at the Health Board, said this year's flu season can be characterized as a prolonged one. "Illness began increasing as usual at the end of December, but the number of flu cases stalled during the first week of January. The season didn't reach its peak until early March. At its highest, the weekly increase in cases was 33 percent, but in the following weeks, the numbers either declined, held steady or grew by 10 to 20 percent," she said.

One possible reason, according to Sõber, was the overlap between the flu's most intense growth period and the Christmas holidays and school break. "Because of their numerous social contacts, children are considered key spreaders of the flu virus. The school holidays acted as a kind of natural isolation that helped flatten the curve and limit the spread of the virus," the epidemiologist explained.

Sõber noted that three flu virus subtypes are currently circulating: A(H1N1)pdm09, A(H3N2) and B. "At the start of the season, the dominant strain was A(H1N1)pdm09. Illness caused by this virus is generally milder but spreads most easily among children. In February, the distribution of circulating strains shifted — with A(H3) and B-type viruses becoming more prevalent, while A(H1N1)pdm has declined."

According to Sõber, this change in the dominant virus strain may be one reason the number of cases has remained high. The rise in B-type flu cases slowed the overall increase in illness, as this strain generally has a lower transmission potential. As a result, the number of new flu cases has grown more slowly, she explained.

"Despite its lower transmission rate, B-type flu causes clinical symptoms and severity comparable to A-type flu. Because of its continued spread, both the number of flu cases and hospitalizations are expected to remain relatively high for several more weeks," Sõber said.

Even so, this flu season has been milder than last year's, she added — with lower illness rates, hospitalizations and mortality. However, since the season is still ongoing, no final assessment can yet be made. "Based on overall illness indicators, this season can be considered average. According to data from the Health Information System, a total of 1,086 people have been hospitalized since the start of the flu season," Sõber said.

"Of those, 47.1 percent were 65 or older and the median age of hospitalized patients was 76. Among all hospitalized patients, 8.9 percent had been vaccinated. Intensive care was required in 9.4 percent of flu-related hospitalizations, with 77.5 percent of those patients aged 60 and older," she added.

According to preliminary data from TEHIK (the Health and Welfare Information Systems Center), 59 people have died from the flu so far this season. The median age of the deceased was 85.7 years and 35 of them — about 36 percent — were over the age of 80. "In addition to their advanced age, all of them had underlying conditions, most commonly cardiovascular diseases," said the head of the epidemiology department.

The effectiveness of the flu vaccine largely depends on how well the strains in the vaccine match those circulating during the season. Among healthy adults, vaccine effectiveness typically ranges from 40 to 60 percent. Some studies have found that healthy adults contract the flu every five to ten years on average. In broader terms, this means the vaccine reduces the annual infection risk from 10-20 percent to 4-8 percent.

In other words, while the vaccine lowers the risk of falling ill, it doesn't necessarily prevent infection altogether. "However, vaccination does significantly reduce the number of severe cases requiring hospitalization and flu-related deaths — by as much as 90 percent," Sõber emphasized.

"Because vaccine preparation must begin by March at the latest to be ready for the next season, sometimes the match between vaccine and circulating strains isn't perfect. This season, the vaccine aligns well with the A(H1N1)pdm strain but is less effective against A(H3). Since the season is still underway, a final evaluation will come later," Sõber concluded.

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