It's the time of year when you can't go to the grocery store, drug store, or even Target without someone nagging you to get your annual flu shot. But this year, doctors are warning that this year's flu shot might not protect you against the bugs we're presently seeing across the country.

Here's a grossly simplified explanation of how the flu shot sausage gets made: every February, a "panel of experts" looks at data on which common flu strains are presently circulating, and uses that data to predict what three to four flu strains will be hot and cool and in and now when flu season rolls around that fall.

Some years, the prediction is more accurate than others. Last year, the vaccine was 50 to 55 percent effective, the U.S. Centers for Disease Control and Prevention says, which is considered a good outcome.

But this year we might not be so lucky. According to an alert issued by the CDC on Wednesday, 52% of the 85 influenza virus samples they've collected and analyzed this winter have been different than the virus strains that were included in this year’s vaccine, suggesting to them that the strain has mutated since the February pow-wow.

The problem strain is the influenza A (H3N2) one. That's the flu that was expected to be the biggest one this year, and, sure enough, it's the most commonly reported flu so far this year. According to CDC director Dr. Tom Frieden, though, they're now seeing a new, mutated version of H3N2: "not something that’s been around before,” he told the Associated Press.

When faced with a mutant virus, the vaccine might decrease the severity of the illness (if contracted), but is unlikely to offer complete protection, the CDC says. Therefore, “though we cannot predict what will happen the rest of this flu season, it’s possible we may have a season that’s more severe than most,” CDC director Dr. Tom Frieden said in a statement.

All that aside, the CDC is still urging you to get the flu shot, as some protection is better than none. On average, about 24,000 Americans die each flu season, and the CDC says that a very high percentage of those who die weren't vaccinated.

But with a weaker shot than in years past, the CDC is telling doctors not to mess around with the infected. Children younger than 2, adults 65 and older, and people with asthma, heart disease, weakened immune systems or certain other chronic conditions are at higher risk, and should head to the doctor immediately if they start showing flu symptoms, even if they got the shot.

So, even though we just told you how your flu shot might not work, based on the CDC's assessment we're still going to leave you with the San Francisco Department of Public Health's list of where to get immunized. You can also read the CDC's alert about this year's mutant strain here, and read all their flu info here.