The second week after school starts, kids with respiratory illnesses—everything from simple colds to asthma attacks—fill my clinic. As a pediatrician I expect this. The timing is right for incubation of the viruses shared by the kids after returning to their classrooms.

But this year was different. Many of the kids had atypically high fevers and body aches with their coughs and congestion. They had influenza.

September is very early for flu. The season usually starts around the winter holidays and peaks between January and March. With such an early start, we braced for a bad year. Now, we are in the midst of an apocalyptic season, leading my colleague Ryan White, content editor for the USC Annenberg Center for Health Journalism, to coin the term “flu-pocalypse.”

The Centers for Diseases Control and Prevention, CDC, has declared this flu season to be the worst in a decade. By the third week of January, influenza activity was widespread in 49 states, including California.

In the past month, nearly one in 10 deaths in the country were attributable to pneumonia and influenza. According to California Department of Public Health, 127 flu-related deaths for people younger than 65 have occurred statewide since September.

Nationwide, 63 children have died during the current epidemic and 80 percent of those children had not received a flu vaccine. Annually, between 37 and 170 child deaths are attributed to the winter influenza epidemic in the U.S.

An epidemic is defined as an increase, often sudden, in the number of cases above what is normally expected. A pandemic is defined as an epidemic that affects a large number of people and has spread over several countries or continents. Pandemics are usually caused by novel strains, because no one has immunity to them. This flu season is an epidemic year, not a pandemic.

Comparing flu seasons year-to-year is a bit unfair. It’s similar to comparing red apples to green apples—both are apples but they have different tastes and crunchiness. Yes, the viruses are all influenza, but different strains have different qualities. This year’s A/H3N2 strain is known for causing more severe illness, as well as its ability to evade vaccine immunity.

In addition to flu A, there are also B, C and D strains. Strains A and B cause the annual human epidemics. C strains usually cause mild human illnesses and D strains cause illnesses in animals. Flu A strains can also infect several animal species, including birds and pigs, which is an important contributor to emerging novel strains.

Influenza A viruses are broken down into subtypes based upon two surface proteins, hemagglutinin (H) and neuraminidase (N). These proteins are part of the name and they are important for the viruses’ virulence, as well as our immune response to the viruses. The influenza A viruses mutate, often leaving our immune systems to deal with new strains from year to year.

Annual flu vaccines are needed to update our immunity to the changing viruses.

Contrary to popular belief, one cannot get the flu from the flu shot.

Injectable influenza vaccines (shots) are prepared with purified proteins, not a whole virus.

After the shot, some people have achiness or low-grade fevers. This occurs because the immune system is responding to the vaccine. Some people have illnesses after the shot, but often those are caused by other winter viruses and it’s just a coincidence.

Nasal flu vaccines contain live viruses that are weakened. Recipients can get a flu-like illness after this vaccination, though its usually milder.

Flu vaccines contain 3 flu strains (two A strains and one B strain) or 4 strains, which includes an additional B strain. The vaccine strains are chosen by national health officials based upon global surveillance of flu activity and then trying to predict which strains will invade the U.S. During a season, multiple flu strains can circulate; thus, a person can have more than one case of the flu.

In some seasons, the predicted strains don’t match the strains that circulate. With a mismatch, the vaccine isn’t as helpful. This year’s circulating A/H3N2 matches the strain in the vaccine, however, the vaccine’s effectiveness is only 17 percent, according to Canadian researchers. An Australian study also found low effectiveness. This may be due to mutations in the vaccine virus that occurred during production.

Annual flu vaccination is recommended for everyone 6 months and older unless they have a contraindication. Some groups, such as infants, young children, pregnant women, the elderly and people with chronic health problems have greater risks for flu-related morbidity and mortality.

However, influenza can cause complications and death in anyone, including healthy people, as this year has again proven. Fortunately, no deaths have occurred among my patients. But we have seen plenty of children with common complications, including ear infections, dehydration and pneumonia.

The good news for California is that, though flu activity is still elevated, it seems to be decreasing. Still, health officials are reminding Californians that it’s not too late to get vaccinated. Although flu vaccines aren’t perfect, they are the best way to protect yourself and those around you against infection, against death if you do get infected and possibly against strains that may later circulate.