The answer to the first question lies within the latter. Flu vaccines are recommended ahead of whenever flu virus activity and infections are on the rise, which is typically when the weather gets cold. For example, in the U.S., flu season dates can last from October through May, usually peaking between December and February, according to the Centers for Disease Control (CDC). Furthermore, the strains of flu virus that are most prevalent change from year to year, which is why new flu vaccines must be formulated almost annually.

The reason that vaccination campaigns begin as early as late August is because it actually takes a while for the flu shot to kick in. “It takes about two weeks after vaccination for antibodies to develop in the body,” according to Vaccines.gov. “In the meantime, you are still at risk for getting the flu. That’s why it’s better to get vaccinated early in the fall, so you are protected before flu begins spreading in your community.”

Even if you don’t get a flu vaccine before October, as is considered ideal, public health officials recommend that everyone—including pregnant women in any trimester—aged 6 months and older, as well as those with compromised immune systems such as small children and the elderly, get their seasonal flu shots in order to protect both themselves and those around them.

While Americans are pretty bad at getting their flu shots, public health experts tout their efficacy. Flu vaccine effectiveness can vary widely from year to year since scientists essentially have to try and predict which strains will be most prevalent, but is generally found to reduce flu illness risk by 40% to 60%. The most common side effects are soreness around the injection site, headache, fever, nausea, and muscle aches, and are generally mild.

But why do we have a flu season in the winter and not the summer? The answer to that is a little bit more unclear—but there are several theories. One common thesis is that lower humidity levels and a chillier climate allow the virus to thrive. Studies have shown that a drop in absolute humidity is correlated with a significant increase in the presence of active flu virus in aerosolized droplets. Furthermore, fewer sunlight ultraviolet rays which may kill the pathogen and cold temperatures may let influenza linger both in the air and on common household surfaces; drier air may also make it easier for the virus to infect the lungs.

And then there’s the fact that the fall marks both the beginning of the U.S. school season and generally drives people indoors, allowing for far more contact—and the potential for flu transmission.

That’s not to say you can’t get the flu in the summertime. But a flu vaccine from the previous season may not protect you during the off-season since influenza virus can mutate so much every year.