First came the cold. Now brace your­self for flu.

Hospital ad­mis­sions re­lated to the flu vi­rus jumped sharp­ly in Minnesota dur­ing the week be­fore Christ­mas, and now state health of­fi­cials are re­mind­ing those who ha­ven't been vac­ci­nat­ed — es­pe­cial­ly the el­der­ly and those with im­mune sys­tem de­fi­cien­cies — that it's time to act.

The spike in cases struck rel­a­tive­ly late in this year's 30-week flu sea­son, but that's not nec­es­sar­i­ly a good pre­dic­tor of how se­vere it will be, said Claudia Miller, man­ag­er of the state Health Department's sec­tion on im­mu­ni­za­tion, tu­ber­cu­lo­sis and in­ter­na­tion­al health. The se­ver­i­ty won't be known for a few more weeks, she said.

"We are see­ing an up­tick. We are still well un­der the lev­els of in­flu­en­za we were see­ing a year ago to date," Miller said. "But once we start see­ing this kind of an up­tick, flu u­su­al­ly spreads pret­ty quick­ly over the next cou­ple of weeks."

Fewer than five pa­tients a week have been hos­pi­tal­ized with the flu in Minnesota over the first nine weeks of the sea­son, which be­gins in Oc­to­ber and runs through April. But those num­bers have since jumped, hit­ting 36 in the week be­fore Christ­mas. Once the num­bers start climb­ing like that, it's not un­u­su­al to see them double for sev­er­al weeks in a row, re­cords show.

The FluView map from the U.S. Centers for Disease Control and Prevention shows high lev­els of in­flu­en­za in Texas, Lou­i­si­an­a, Mis­sis­sip­pi, Al­a­bam­a, Mis­sou­ri and Oklahoma, but mini­mal lev­els in Minnesota and the sur­round­ing states in the Upper Midwest.

Miller said the vi­rus is hit­ting young­er and heal­thy adults hard in oth­er states, but said of­fi­cials won't know for a cou­ple of weeks wheth­er that will be the case in Minnesota.

Most of the 140 mil­lion or so doses of flu vac­cine this year are de­signed to work against three strains of the vi­rus, ac­cord­ing to the CDC, but about a quar­ter are de­signed to work against four strains: two type A and two type B. The vac­cines gen­er­al­ly take ef­fect with­in a cou­ple of days.

Miller said the vac­cine cov­ers the most prev­a­lent strain of flu turn­ing up in Minnesota. But oth­er fac­tors, in­clud­ing an in­di­vid­u­al's im­mune sta­tus, in­flu­ence its ef­fec­tive­ness. "It is a good match, but not an ab­so­lute as­sur­ance of pro­tec­tion," she said.