When I went to a pharmacy in Brooklyn to get a flu shot last year, I was presented with a choice: one vaccine with three different strains of the flu virus for about $30 or, for just $10 more, four strains. It sounded vaguely like a late-night television infomercial.

I stood at the counter, confused. Didn’t I want every strain? I thought that one new vaccine was developed each year and that it was more effective some years than others. What was I missing?

As it turns out, the choice I was given was not unusual . Until a few years ago, the typical flu shot included three strains of the virus in what is known as a trivalent vaccine: two strains of influenza A, one of influenza B. Now, more flu-shot makers are adding a second B strain to create a so-called quadrivalent vaccine that provides a bit more protection.

The more strains the better, right? I paid for the additional B strain. The vaccine experts I spoke with later said that was a good choice for me, but that didn’t mean it would be better for everyone based on what’s available now. The Centers for Disease Control and Prevention said it did not prefer one vaccine over another. Its guidelines are based largely on age and other characteristics (like pregnancy or chronic illness).