This post has been updated.

This year's flu season is turning out to be so intense that the number of people seeking care at doctors' offices and emergency rooms is almost as high as levels reported during the peak of the 2009-2010 swine flu pandemic, federal officials have said. It has already caused the most hospitalizations in nearly a decade,  they said. During the week ending Feb. 10, 22 children's deaths from flu were reported to federal health officials, bringing the total pediatric deaths so far this season to at least 84 nationwide.

But the latest report from the Centers for Disease Control and Prevention also show signs that this harsh flu season may be starting to level off.  The percentage of doctor visits for fever, cough and other flu symptoms was the same during the week of Feb. 4-10 as the week before — about 1 in every 13 visits.

The figures are based on updated data and now show the percentage of Americans seeking medical care for such symptoms to be just slightly lower than those during the swine flu pandemic. The week ending Feb. 10 was the first one in which that indicator did not increase. Officials say this season has been unusual because flu has slammed virtually the entire continental United States at the same time, and that intensity has continued for several consecutive weeks.

Reporter Lena H. Sun has been talking to health officials about this flu season. Here are some answers to frequently asked questions:

Q: What's going on with the flu this season?

This flu season started early, and the entire country is experiencing widespread and intense flu activity. The data show there's been a very rapid increase in the numbers of people going to see their doctors or health-care providers, affecting hospitals, schools and workplaces. In California, hospitalization rates are four times higher than what they were in 2014-2015; in Oregon and Minnesota, hospitalization rates are double what they were back then.

Q: Is there something particularly bad about this year's flu strains?

This season, the predominant strain is also the nastiest, H3N2, which causes the worst outbreaks of the two influenza A viruses and two types of influenza B viruses that circulate among people and are responsible for seasonal flu epidemics each year. Seasons when H3N2 strain dominates are associated with more hospitalizations, more deaths and more illnesses. Those who are particularly hard hit are the very young, the elderly and people with certain chronic health conditions, experts say.

As top CDC flu expert Daniel Jernigan said: “Of the viruses we hate, we hate H3N2 more than the other ones.” This strain, which has been around for 50 years, is able to change more quickly to get around the human body’s immune system than the other viruses targeted in this year’s seasonal flu vaccine.

The CDC estimates that flu has resulted between 9.2 million and 35.6 million illnesses and 12,000 to 56,000 deaths each year in the United States since 2010.

Q: Are there any differences in flu symptoms this season?

No. Flu symptoms vary from person to person. In general, people who have the flu often feel some or all of these symptoms, according to the CDC:

  • Fever or feeling feverish/chills (not everyone with flu will have fever)
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue (tiredness)
  • Vomiting and diarrhea, although this is more common in children than adults

Most people who get the flu get better in several days to less than two weeks. But some people can develop serious complications caused by viral infection of the nasal passages and throat and lungs. Young children, adults ages 65 years and older, pregnant women, and people with certain chronic medical conditions are among those groups of people who are at high risk of serious flu complications, possibly requiring hospitalization and sometimes resulting in death.

Q: How effective is this year's vaccine?

The CDC and clinicians recommend a yearly flu vaccine for everyone 6 months of age and older as the first and most important step in protecting against this serious disease. The body takes about two weeks to produce a full immune response. Flu vaccination can reduce illness and prevent flu-related hospitalizations. A study in Pediatrics last year was the first of its kind to show that vaccination lowered the risk of flu-associated death by half among children with underlying high-risk medical conditions and by nearly two-thirds among healthy children.

But flu vaccines aren't perfect. Even in a good year, the flu vaccine isn’t as good as most other vaccines. Health officials must choose the influenza strains that vaccine makers should target for an upcoming season months in advance, when it is hard to know what strains might be circulating. When flu vaccines are well matched to circulating viruses, effectiveness is, at best, about 60 percent. (The measles vaccine, by comparison, is about 97 percent effective with two doses.) In a year when the circulating flu strains closely match the vaccine, that effectiveness rate means that about 3 in 5 people who get shots are far less likely to become so sick that they require a visit to a doctor.

In general, flu vaccines tend to work better against influenza B and influenza A (H1N1) viruses and offer lower protection against influenza A (H3N2) viruses.

The CDC has released a report showing the flu vaccine at mid-season is overall 36 percent effective, but falls to 25 percent against the dominant H3N2 strain. Its 25 percent effectiveness rate means that one in four people who get the shot reduce their risk of becoming sick enough to need to see a doctor. The report confirms what federal health officials and infectious-disease experts have suspected for some time. In unrelentingly bad flu seasons such as the current one, which is dominated by the most dreaded flu strain, vaccines are less effective.

The CDC report found that the current vaccine is 67 percent effective against the other influenza A strain, H1N1, and 42 percent effective against influenza B viruses.

One piece of encouraging news: In children younger than 9, it offers much greater protection against the predominant strain, reducing by more than half the risk of becoming so sick that they need to see a doctor.

Q: How long will this flu season last, and when will it peak?

Flu is very unpredictable. The exact timing and duration can vary, but influenza activity often begins to increase in October. Most of the time, flu activity peaks between December and February, although activity can last as late as May. Experts aren't sure when this season will peak. Officials were hoping the season had peaked in the first week of January, but the latest data shows activity continued to increase into the fourth week, with unrelenting intensity, which is unusual.

Even if flu activity peaks soon, there will be many more weeks of flu activity based on patterns of similar seasons when this particular strain has dominated. In those seasons, flu season lasted for at least 11 to 13 more weeks after activity hit the peak. The overall hospitalization rate for the week ending Feb. 10 was almost 68 people per 100,000, higher than the 64.2 per 100,000 in the especially severe 2014-15 season.

Q: What flu symptoms should parents watch for in their children?

Patsy Stinchfield, a pediatric nurse practitioner and senior director of infection prevention and control at Children's Hospitals and Clinics of Minnesota, said parents should call or bring their children to see the doctor early if the children aren't drinking, are really lethargic, have pain in a specific place in their chest or are having difficulty breathing after normal activity, such as going upstairs. Also, if they are getting better and suddenly get worse, bring them in to the hospital right away, because this could be a sign of a secondary bacterial infection.

“These kids don't do well when brought in late for care,” she said.

These stories about a 6-year-old and a 10-year-old who died recently of flu show how dangerous the disease can be.

Q: Why are so many otherwise healthy children dying from flu?

Children’s vulnerability is of special concern this season. As in past winter flu seasons, about three-quarters of children who have died were not fully vaccinated, and about half were previously healthy, officials said. The number of pediatric deaths probably does not include all cases because of the typical lag time for reporting non-hospital deaths. (States are not required to report adult deaths.)

Based on current trends, the number of child deaths could exceed the 148 reported during the 2014-15 season, when the vaccine’s poor match resulted in an overall effectiveness of 19 percent. This season’s flu vaccine is almost twice as effective overall, at 36 percent.

Experts say one reason healthy children are much more vulnerable has to do with how their immune system responds. For some who haven’t received a flu shot, infection with a flu strain they haven’t previously been exposed to can trigger their immune system to overreact. That can lead to widespread inflammation that is ultimately fatal.

Q: Some news reports have claimed the flu vaccine is expected to be only 10 percent effective this year; is this true?

U.S. officials often look to Australia, where the flu season begins during our summer and their winter, for clues on what to expect for flu in the United States. Flu virus infections began increasing earlier than usual in Australia, hitting historic highs in some areas, and interim reports suggested that vaccine effectiveness against H3N2 was only 10 percent. The vaccine for the United States has the same composition as the one used in the Southern Hemisphere.

But Australia vaccinates only the elderly and infirm, who generally don’t respond as well to vaccines. Its approach makes a difference in vaccine effectiveness rates because children have better responses. CDC officials said a better predictor of vaccine effectiveness for H3N2 is last season's vaccine's effectiveness in the United States. The current vaccine contains the same H3N2 component as last season, which was about 32 percent effective.

A recent study by Canadian researchers suggests the H3N2 component of the vaccine is only about 17 percent effective in preventing infection in parts of Canada.

One reason the vaccine may be less effective on this strain has to do with the way most vaccines are manufactured: by growing them in eggs. When the H3N2 virus is inserted into eggs, tiny changes take place — more than with other viruses — that affect the ability of the egg-grown vaccine to target the circulating strain, which appears to be a factor in vaccine effectiveness.

Q: Should I still get a flu shot?

Yes. Experts say there are other strains of flu that are showing up and are a major cause of disease. Influenza B viruses, which this year's vaccine also covers, are increasing and now make up about 30 percent of the viruses being tested. Influenza B viruses tend to emerge later in the season. One of the other influenza viruses, H1N1, a type of influenza A virus, is appearing in states that have already had lots of H3N2 activity. Even a partially effective vaccine offers some degree of protection against serious complications. Here is a vaccine-finder widget that can find locations that still offer the flu shot.

Q: What if I think I have the flu? What kind of treatment is available? 

Antiviral drugs such as Tamiflu can lessen the symptoms and shorten the duration of illness. Most people who get the flu have mild illness and don't need antiviral drugs. But people who are very sick or people with flu symptoms who are at high risk for serious complications should be treated as soon as possible with antiviral drugs. That means people 65 and older, young children, people with chronic conditions, such as diabetes, heart disease or asthma, and pregnant women and others more vulnerable to serious flu illness.

The CDC has been urging clinicians not to wait for confirmed testing but to begin treatment if they suspect flu in a severely ill or high-risk patient. Young children and children with high-risk conditions respond to Tamiflu best when it's given in the first 48 hours.

Also, if you or your children are sick, stay home from work or school to avoid spreading it to unsuspecting others, especially people who have weak immune systems and are at higher risk.

Q: What does flu do to the body, and why do some otherwise healthy people die?

The virus causes damage all by itself, but the immune response it triggers, especially among young, healthy adults, also makes people sick and can lead to death. After entering the body, often inhaled through the nose or mouth, the influenza virus begins hijacking human cells in the nose and throat to make copies of itself. The virus implants in the mucous membranes at the back of the throat and upper part of the bronchial tubes. In doing so, it weakens or destroys the feather-like protective cells in the bronchial tubes that push out bacteria and other material, “like so many little oars,” said William Schaffner, an infectious disease expert at Vanderbilt University.

Without that protection, bacteria — including the kind that causes pneumonia — can work their way down the bronchial tube and into the lungs, causing complicating bacterial pneumonia, he said.

Influenza virus can also trigger such a massive immune response, especially in adolescents and younger adults with robust immune systems, that these otherwise healthy  people are basically drowning in their own immune cells and proteins, said Stinchfield at Children's Minnesota. Or, as Schaffner put it, the  overwhelming immune response “can make your whole body’s metabolism go awry. It’s like an army set off bombs and killed all the good stuff while targeting the virus.”

Q: If someone has already had the flu this season, does that mean they now have immunity to the strain they had and won't/can't get it again this season?

While it's likely that a person would be protected from illness caused by the virus that first made them sick, there are four different flu viruses that are circulating this season, according to the CDC. They advise you to take preventive measures to protect yourself, including vaccination if you haven't already received your vaccine.

Someone can definitely get flu more than once during a season. This 58-year-old Texas mother and grandmother woman survived her first bout of flu, but not her second.

Q: How long do flu germs stay active?

They can live on surfaces for 1-2 days.

Q: What can I do to avoid getting sick?

Aside from vaccination, the single most important thing to do is prevent the spread of germs. Wash your hands frequently, soap up between your fingers and don’t forget your thumbs, and scrub for about 20 seconds. Some other good tips, as reported by my Washington Post colleague Jura Koncius:

Isolate toothbrushes. Once a household member has a cold, it’s best to separate their toothbrush from the rest of the pack to avoid germ contamination. After the person gets well, replace the toothbrush or toothbrush head with a new one.

Launder bedding frequently. The best thing to do if someone is sick is to put them in a separate room to sleep, preventing the spread of germs as well as preserving your precious sleep. If this isn’t possible and you must share the same bed, wash your sheets frequently in hot water. It even helps to wash just the pillowcases every few days. Bath towels should be washed every day or two.

Clean your humidifier. Humidifiers help soothe sinuses and nasal passages, especially when you have coughing and chest congestion. But if you are using your humidifier on a daily basis to make your patient feel better, once a day you should empty the tank and then wipe it down until it’s dry. Do a thorough cleaning of it at least weekly, following the directions provided for your particular model. “You don’t want germs breeding in there and spewing out into the room,” Carolyn Forte, director of the Good Housekeeping Home Appliances, Cleaning Products, and Textiles Lab, told The Post.

Keep the remotes wiped off. Flu sufferers who are home and clicking through Netflix and HBO offerings need to remember that all those remotes will be covered in germs. Keep a container of antibacterial wipes around so you can clean the remotes on a daily basis; clean your light switches and doorknobs with them, too. Put a small wastebasket in the TV room with a plastic liner for all the wet tissues and used wipes, and empty it once or twice a day.

Read more:

Children's deaths from flu rise sharply amid signs that season may be hitting a plateau

This season's flu vaccine is only 36 percent effective, but say you should still get a flu shot

Why this may be a bad flu season

Long reads:

Chasing a killer: US and Congolese scientists are tracking the deadly monkeypox virus

Fueled by drug crisis, U.S. life expectancy declines for a second straight year

Brigitte Adams caused a sensation when she froze her eggs. Then she tried to use them.