How did COVID-19 impact flu in Stanislaus County? Here are the numbers
No deaths from influenza have been reported in Stanislaus County through March 21, according to county public health surveillance.
For the current flu season from September 2020 through March, only five of the more than 15,000 samples tested were positive for an influenza strain.
Such statistics are uncommon.
The feared “twindemic” of influenza and COVID-19 striking simultaneously didn’t happen in the county, the country or globally. And, as we move further into spring, a surge of seasonal flu is unlikely.
Since the 2016-2017 flu season, 8 to 24 deaths, including one to three children younger than 18, have been reported annually to Stanislaus County public health.
In the state this season, 44 people and no children have died due to influenza, according to the California Department of Public Health. This is compared to 889 deaths during 2019-2020 and 1,648 during the 2017-2018 severe season.
“While it is hard to define causation, we do know that the mitigation strategies that we implement for COVID are also effective against other respiratory illnesses like influenza,” said Chelsey Donohoo, epidemiologist for Stanislaus County Health Services Agency in an email.
The Centers for Disease Control and Prevention attribute the paucity of influenza, as well as lower rates of other winter respiratory viruses, to the mitigation efforts against the SARS-CoV-2 coronavirus. Staying home, especially when ill, frequent hand washing, physical distancing and wearing masks have proven effective for limiting the spread of influenza.
An obvious question then is: “Why didn’t those measures have the same disruption on the COVID-19 pandemic?”
The answer isn’t completely understood, but many factors seem to contribute. Notably, the influenza and coronaviruses are different and how human hosts respond to them is also different.
The SARS-CoV-2 virus is more contagious than the influenza virus.
R0, pronounced R nought, numbers measures a germ’s ability to spread to others. For influenza the R0 is 1 to 2, meaning an infected person can spread the flu to one to two more people. The R0 for SARS-CoV-2, is estimated to be two to three, so about twice as transmissible.
Although both are spread by large respiratory droplets, such as from coughing and sneezing, from contaminated surfaces and by smaller aerosolized droplets, SARS-CoV-2 may spread more easily by aerosols, again making it easier to spread.
Human infection: Influenza vs. SARS-CoV-2
The two germs have different effects once they invade the human body, though they result in some overlapping symptoms, including cough, difficulty breathing and fever.
Differences include incubation time, asymptomatic infections and pre-existing immunity.
The incubation period, the time from exposure until infection, for the coronavirus averages four to five days, but can be as long 14 days. Once infected, people with the SARS-CoV-2 can spread the disease without being ill. Such asymptomatic spread makes the virus more difficult to track and contain.
The incubation period for influenza is much shorter, one to four days. Once infected, most people have abrupt onset of symptoms and they feel sick so many stay home, a self-imposed isolation. They may shed the virus from one day before getting sick until about seven days after. Asymptomatic infection with influenza is thought to be less common and plays a minor role in its spread.
No humans had immunity against the SARS-CoV-2 virus at the beginning of the pandemic, since it is a novel strain.
For influenza, nearly all individuals older than 8 have had flu at least once or have been vaccinated. Immunity induced by some strains can provide limited cross protection to other other strains, which may help limit spread. However, this is far from complete. Thus, the need for annual flu vaccines to protect against newly emerging flu strains.
More adults were immunized against influenza this season: about 53% of adults in the U.S. got flu vaccines, compared to 45% for the 2019-2020 season, according to the CDC.
Other reasons for less seasonal flu
Influenza viruses had fewer opportunities to spread with minimal international travel and fewer person-to-person interactions, with work-from-home and school closures worldwide.
Another factor was that fewer people sought medical care, including for respiratory illness, due to fear of catching COVID-19 in a medical facility. This contributed to a 61% decline in nasal samples submitted for flu testing in clinical labs in the United States.
But based on routine monitoring, a 98% decline of influenza viruses circulating was noted in the U.S., and declines were also seen in the World Health Organization sentinel labs across the globe.
School-age kids are a major reservoir for influenza and spread throughout the community, and this was disrupted with closure of school campuses. In contrast, school children contribute less to transmission of SARS-CoV-2, even within their families.
Some mitigation efforts for the pandemic, such as wearing masks and hand washing, have become integrated into life and may not go away immediately, if ever. Health experts, including Dr. Anthony Fauci, are skeptical that handshakes need to return as common greetings.
It is to be seen if these “new normal” practices continue to keep respiratory viruses in check with return of more routine activities.
This story was produced with financial support from The Stanislaus County Office of Education and the Stanislaus Community Foundation, along with the GroundTruth Project’s Report for America initiative. The Modesto Bee maintains full editorial control of this work. To help fund The Bee’s children’s health and economic development reporters with Report for America, go to bitly.com/ModbeeRFA
This story was originally published April 8, 2021 at 4:00 AM.
