By Miriam Berger
Washington - A "twindemic" of Covid-19 and influenza is a recipe for public health nightmares.
Strict measures to control the spread of the coronavirus were expected to prevent flu transmission, which appears to have largely held true for 2020. But this flu season, some countries are on track to be hit much harder, while others could be spared.
Efforts to track flu cases face obstacles, as flu tests are not ubiquitous and the illness can be confused with others, including Covid-19. But many healthcare systems track hospitalizations for flu-like symptoms.
The United States, experts warn, could soon be facing its first "twindemic" season. Last winter, flu cases in the United States hit record lows as the coronavirus surged. This year, flu cases are rising alongside the highly contagious omicron variant that's already overwhelmed hospitals. Europe's flu season is also just starting - and likewise expected to be worse this year.
Other places, including Israel, are seeing flu spikes this winter after historically low case levels last year. Yet the trend looks different in Australia, which had low flu cases last year - and even fewer this summer, when the country's flu season usually hits.
And some countries, such as Brazil, are battling out of season flu outbreaks as they brace for the full effects of the fast-spreading omicron variant.
Israel recorded no influenza cases during its 2020 flu season. This year, hospitals are nearing capacity with flu cases just as coronavirus cases are starting to climb.
As of early December, Israel had recorded around 280 flu hospitalizations since September, when the season began, about double those seen during that same period in 2019, the last ordinary flu season before the coronavirus pandemic. By mid-December, 565 people had been hospitalized with the illness, according to the Israeli Center for Disease Control. Of those, 239 were in paediatric wards and 39 in maternity wards.
By the month's end, the Ministry of Health said 1 849 people were hospitalized with the flu, including 605 children and 124 pregnant women.
Steve Walz, the spokesperson for Sheba hospital, which is Israel's largest, said in late December that the hospital was "operating at almost 100% capacity" due to flu cases, while, at that time, he could count "on one hand" the number of people hospitalized for Covid-19.
That could start to change, with new coronavirus cases in Israel quadrupling over the past 10 days. This week, Israel also recorded its first case of "flurona," a patient simultaneously infected with both the flu and the coronavirus.
Compared to last winter, mask use and social distancing are far less widespread in Israel, which could partly account for the flu's resurgence. Though the country in late November banned international travel because of the omicron variant, in general in recent months its borders have been more open than in the lead-up to winter 2020.
Experts also point to a decline in flu vaccine uptake, despite the country's high rates for vaccination against the coronavirus. About 17% of Israel's total population received their flu jab so far this year, compared to approximately 24% in the same period last year and 20% in 2019, according to the Ministry of Health.
Australia kept its borders shut to travellers for much of the past two years. That's also likely helped keep the flu at bay.
Australia's flu season typically ranges from May to August, the southern hemisphere's winter months. In 2019, the country recorded some 950 flu deaths and 313 033 infections, surpassing the five-year average, the Australian Broadcasting Corporation reported.
Infections nosedived in 2020, when Australia recorded around 20 000 flu cases and 37 deaths.
But then flu infections dropped even further in 2021, with Australia recording less than 500 cases and no related deaths that season. Australia's Department of Health logged just one hospitalization.
The decline in cases was recorded despite a slight drop in flu vaccination rates in 2021, with 74% of the population receiving the shot in 2021, 82% in 2020 and 77% in 2019, according to the Ministry of Health's flu tracker.
One explanation could be Australia's coronavirus-related restrictions on travel to and within the country, which "likely disrupted external and local seeding" of the flu virus, World Health Organization spokeswoman Margaret Harris said in an email.
Ian Barr, the deputy director for the WHO Collaborating Center for Reference and Research on Influenza based in Melbourne, told the ABC that the majority of this summer's flu cases were recorded in quarantine centres set up for incoming international travellers.
"The whole influenza world is topsy-turvy at the moment," he said.
In December, officials in Rio de Janeiro started setting up mobile clinics to deal with influenza patients flooding emergency departments and doctors' offices. By mid-December, the city recorded more than 21,000 infections - a more than 2,600% increase over just three weeks, according to the Brazilian Report.
Demand for flu vaccines also rose, and the city couldn't keep up: It temporarily suspended its vaccination program as it waited for the Butantan Institute in Sao Paulo to send an emergency 400 000 million doses, Brazil's state media reported.
Days later, São Paulo reported that a similar flu strain was behind its rising rate of emergency room visits and hospital admissions. Nancy Bellei, the coordinator of testing at São Paulo Hospital, told Brazilian media in mid-December that the rate of flu hospitalizations over the past week had been higher than during the flu's typically peak period from March to June.
Out of season outbreaks are not uncommon in Brazil, especially this year, Harris said.
"Due to low levels of flu virus circulation for the last two years [around the world], it will not be surprising if influenza activity resurges during or out-of-season, as countries adjust their public health measures in response to the Covid-19 pandemic," she said.
Last year flu cases dropped across the southern hemisphere. But this winter's outbreak is particularly worrying for public health officials, because it comes alongside a rise in coronavirus cases.
Since March 2020, about 616 000 people have died of Covid-19 in Brazil, a death toll second only to that of the United States. Nonetheless, Brazilian President Jair Bolsonaro has opposed measures, such as mask-wearing and social distancing, that limit the spread of viruses.
Brazil has yearly influenza vaccine drives that target people above the age of 60, children, and other high-risk groups like health-care professionals. As a result, a relatively high rate of older Brazilians are vaccinated against the flu. The country has also given out enough coronavirus shots to inoculate some 78% of its population, according to Reuters.