So how do they check how well the vaccine is performing?

The annual flu vaccine incorporates three or four viruses which should be, in theory, the ones circulating for that flu season. Thanks to antigenic drift, the flu mutates often, sometimes diverging from the vaccine strains. These are two slightly different measurements.
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Erika:
I got my period 3 times in 3 weeks. Is this a bad sign?
(7 November 2016)

Erika:
But I have to wait for a coupl of days before I take the test, is it still possible that I'm pregnant?
(7 November 2016)

Erika:
My gyno told me it's possibly just because of the pill and then the Plan B but she gave me a BHCG just to be sure.
(7 November 2016)

Erika:
I had unprotected sex the next day and tooke a Plan B one step less than 36 hours later, 6 days later I started bleeding again and this time the pain is worst, it come and goes but still pretty bad.
(7 November 2016)

Erika:
Then I missed a few doses and stop taking it. 5 days later I noticet I had some light spotting, it got more and more till the next morning and I woke up with a sever pain and had a heavy bleeding that lasted for another 4 days.
(7 November 2016)

Erika:
As usual It only lasted 3 days. i started my birth control on the second day.
(7 November 2016)

Erika:
I first got my period on oct 16 when it was due, I had very heavy bleeding but every thing was normal.
(7 November 2016)

Shannon:
And I don't want to be sneaky because they're gonna find out lol.
(7 November 2016)

Shannon:
I feel terrible, I really wanted to see where it could go with this guy, but now I feel like it'd be rude to my friend.
(7 November 2016)

Shannon:
But I had intended on getting together with the guy in the first place.
(7 November 2016)