Rising Number of Flu Cases Raise Public Health Concern Glow Images, Inc / Getty Images Portrait of a young woman suffering from cold and flu In Boston, hospitals are restricting visitors to protect patients with weakened immune systems; some are banning children under 14 years altogether from traipsing through their wards. And hospital personnel are carefully monitoring folks flowing in through their lobbies for signs of sneezing, coughing or fever. The hospitals are wise to take such care because Boston is experiencing a flu crisis. At Massachusetts General Hospital, 536 people have been treated for influenza so far this season, 97 in the last week alone. Last year, just 29 people were admitted for stays of one night or more for flu-related illnesses; this year 167 have already been hospitalized. The current outbreak led the city's mayor to declare a public health emergency when the confirmed influenza caseload hit 700, compared to last year's tally of 70. The announcement allowed the release of thousands of free vaccines for those who haven't been immunized, and an admonition that people who are sick should stay home to prevent spreading the virus. MORE: Pregnant Moms" Flu Linked to Higher Risk of Autism Among Children And Boston isn't the only city confronting rising cases. In Minnesota, hospitals in Minneapolis and St. Paul are opening up extra units to house patients felled by flu. Forty-one states report widespread influenza, and according to the Centers for Disease Control (CDC), about 3 in 10,000 adults over age 65 have been hospitalized for influenza, compared to 1 in 100,000 the same time last year. The elderly and those with weakened immune systems are most vulnerable to serious complications. As the flu season continues to build, public health officials are bracing for what could be the most severe outbreak in recent years. "Certainly last year was mild, and the year before that was relatively mild," says Dr. Michael Jhung, medical officer in the influenza division at the CDC. But the number we are at right now, 5.6% [the proportion of people seeing their doctor for flu-related symptoms] compared to last year's peak of 2.2% tells you this year is more severe than last year for sure. VIDEO: One More Reason to Get the Flu Shot: Your Heart Whether or how high that means the incidence will climb isn't clear yet. Influenza season generally runs from October to April, peaking in February, but this season the flu hit earlier and harder. It's possible that the seasonal cycle is simply peaking earlier, says Jhung, but because influenza is so unpredictable, health officials won't know for sure until the spring. Why is this year's flu so severe? The dominant strain causing the most illness, H3N2, is associated with more virulent symptoms of fever, coughing, and respiratory distress, as well as complications such as pneumonia and bacterial infections. Generally viruses that can lodge deeper in the lower lungs tend to cause more health problems, but scientists are still trying to figure out where H3N2 gets its virulence. Any influenza virus can also cause changes in the lungs that can make people more susceptible to bacterial infections that normally don't pose a problem. VIDEO: Chicken Eggs and Antigens: How the H1N1 Vaccine Is Made Fortunately, the flu vaccine that was distributed in the fall is a good match for the H3N2 strain. Seasonal vaccines, which are re-issued each year, contain three different strains of influenza in order to maximize the chances of protecting against whatever multiple versions of the virus may be circulating. This year's shot contains a strain nearly identical to the current H3N2, as well as an H1N1 strain that is is a near-match for the H1N1 infecting some people, and an influenza B strain that is 80% matched to the version circulating in the U.S. But that could change. Influenza is a notoriously wily virus, and known to mutate quickly and effectively to evade confrontation with immune defenses generated by a vaccine. "It may change in a week, it may change in a month," says Jhung of the circulating virus. MORE: How Fast Could a Swine Flu Vaccine Be Produced? It's also possible that some people who were immunized may still get sick, since no vaccine is 100% effective at protecting against infection. Those who were vaccinated earlier in the season are likely in the best position to fend off the flu, since it takes about two weeks for the body to develop the proper antibodies, once the immune system is prompted by the influenza templates in the shot. But flu vaccination rates are notoriously low; so far, about 35% to 40% of people have been immunized, about the same percentage as last year. Among children older than six months, less than 45% were vaccinated, according to a recent study involving youngsters in three states. The CDC is seeing signs of improvement, however, since more physicians and pregnant women are getting flu shots this year compared to last. Getting immunized, say health experts, is the best defense against illness. "We want to make sure that the public health message is that you should still get vaccinated," says Dr. David Hooper, chief of the infection control unit at MGH. We don't want perfect to be the enemy of the good. Those who are immunized and still do get sick tend to experience milder symptoms, he says, so it's still smart to get a shot. And remember: there are three months left to go in this long and hard flu season.