If you're wondering what happened to the flu, you're not alone.
It's curiously late, like our knee-deep snow, here and across the nation.
This is the slowest start in the nation's flu season in 29 years. And while the number of cases began to pick up this month, it's not entirely clear what that means.
Influenza generally peaks in February and then peters out in March and April, yet it also varies in its timing and duration. The season has been known to start as early as October and last as late as May, according to the federal Centers for Disease Control and Prevention.
"We've been fortunate that the onset of flu has been delayed, but it's upon us now," said Dr. Gale R. Burstein, health commissioner of Erie County.
"It's difficult to say how things will go. The flu is very unpredictable," said Dr. Howard S. Faden, chairman of infection control and director of virology at Women & Children's Hospital in Buffalo.
The pediatric hospital oversees one of two laboratories in the region -- Erie County operates the other -- that test for the presence of flu viruses as part of the U.S. influenza surveillance system.
The hospital's lab normally has confirmed dozens of positive cases by this time of year, a sign that the virus is widespread in the community. As of last week, it had identified none. The Erie County lab had confirmed only three positive samples sent in by doctors and other hospitals, with the first testing positive in late December.
People are suffering from respiratory illnesses, such as the common cold. Until recently, however, it probably wasn't the flu.
There were 132 million doses of vaccine distributed for this season, and mass immunization programs have ended in the Buffalo Niagara region. But public health officials continue to recommend that unvaccinated individuals, especially those at risk of complications from the flu, get a shot from a doctor or health center as a way to protect themselves and to prevent spreading the disease.
Complications of flu can include bacterial pneumonia, ear infections and worsening of such chronic medical conditions as congestive heart failure.
Over 31 seasons between 1976 and 2007, estimates of flu-associated deaths in the United States ranged from about 3,000 to 49,000 people, according to the CDC. An estimated 90 percent of the deaths occur in people 65 and older, but young people are not immune.
>Death of a 15-year-old
What happened to Chelsea Oliver offers a cautionary tale.
The Amherst High School sophomore came down with flulike symptoms Oct. 25, 2009, and, after visiting her doctor, was treated as though she had the flu.
Things only got worse, with Chelsea soon complaining that she couldn't breathe. She was admitted into the intensive-care unit at Women & Children's Hospital, where she was diagnosed with pneumonia and sepsis, an inflection of the bloodstream.
The hospital staff intubated Chelsea and placed her on ECMO, or extracorporeal membrane oxygenation. ECMO is similar to a heart-lung bypass machine and is used when patients fail to respond to a respirator. Under ECMO, the patient's blood receives oxygen from an artificial lung.
Her condition fluctuated over the next few days, and then rapidly deteriorated. She died Nov. 1, less than a week before the first signs of sickness. She was 15.
"It's still hard to talk about it. Everying that happened didn't feel like reality," said Cheryl Craft, her mother. "She had been perfectly healthy. Then she walked into the hospital with me, and that is the last time we spoke to each other."
That season featured a worrisome new strain of influenza called H1N1, also known as swine flu. It turns out that is what Chelsea had, in addition to MRSA, or methicillin-resistant staphylococcus aureus, a bacterium resistant to common antibiotics.
Several weeks after her death, supplies of the H1N1 flu shot arrived after delays nationwide caused by several factors, including production problems.
"I still wonder if her life would have been saved had the vaccine arrived on time," said Craft, patient manager at the University at Buffalo School of Dental Medicine.
Since then, Craft has gone on to become an advocate for flu immunization. She joined the Western New York Pediatric Adolescent Coalition, a group committed to increasing immunization rates.
Last fall, she also established the Chelsea Oliver Foundation in honor of her daughter. Its goals include providing scholarships to high school students interested in the health care field.
"I don't want Chelsea's story to die," Craft said.
It's not possible to count every individual with influenza. Instead, flu trackers monitor such indicators as influenzalike illness reported by emergency rooms and outpatient centers, deaths from influenza and cases confirmed by laboratories that test nasal secretion specimens from patients.
Nationally, the number of respiratory samples testing positive for the virus remained below 10 percent until early February. This happened only once before over the last 29 years -- in 1987-1988 -- according to the CDC. By Feb. 18, the percentage of specimens testing positive for influenza in the United States had risen to 14.4 percent.
The moment when laboratory-confirmed influenza cases rise above the 10 percent level is a key measure public health officials use to indicate the start of flu season.
"The increases we are seeing in the number of respiratory samples testing positive for flu should forecast increases in other flu activity indicators in the coming weeks," Lyn Finelli, chief of domestic surveillance for CDC's Influenza Division, said in a statement.
The CDC continues to recommend that individuals 6 months and older get vaccinated. It takes about two weeks after a shot for the body's immune system to develop antibodies that provide protection against the influenza viruses in the vaccine. Antibodies are proteins that neutralize such harmful substances as bacteria and viruses.
The viruses that cause influenza constantly mutate, changing year to year. That requires manufacturers to regularly update the vaccine based on predictions by experts of what strains will predominate.
As a result, flu vaccine does not ensure total protection. Its effectiveness depends in part on the match between the viruses in the vaccine and the viruses circulating in the community. So far, the majority of viruses identified this season appear well matched to the strains in the 2011-12 vaccine.
>A perplexing question
A vaccination is considered the best way to prevent the flu, but many people don't get it. An estimated 43 percent of Americans 6 months and older were immunized for the 2010-11 flu season, far short of a target of 80 percent in the nation's disease-prevention plan, Healthy People 2020.
Why don't more people get immunized and take other simple steps to prevent the flu, such as regularly washing hands? That's a question that has perplexed public health officials.
"There is no magic answer," said Marc T. Kiviniemi, a social and behavioral scientist at UB who published a report last year on the willingness of the public to engage in influenza precautions.
His survey in BMC Public Health found that despite intense flu education efforts, as well as media attention, many people don't appear to know the most common actions to prevent the disease, such as avoiding sick people and coughing into your sleeve.
In addition, there is great variation in people's willingness to engage in preventive measures, including immunization.
"The ideal would be to engage in behaviors without thinking about them, like brushing your teeth," Kiviniemi said. "But the challenge with the flu is that a lot of the preventive behaviors require you to do things when there is no health risk prevalent."
The 2012 influenza season
*Flu activity overall is increasing, but numbers remain low, especially when compared with past years.
*Visits to doctors for influenzalike illness also remain low.
*The proportion of deaths attributed to pneumonia and influenza has increased but is below the level expected for this time of year.
*The majority of flu viruses circulating so far are well-matched to the strains in the 2011-12 influenza vaccine.