There were more than 21,000 people affected with whooping cough last year, most of which were teens and children in the United States.

The total number of cases is the highest yet since 2005 and is among the worst years beyond half a century, U.S. health officials informed on Wednesday.

The health officials are truly baffled by the abrupt rise in cases, since whooping cough vaccines are very effective in children, and costs of vaccinating kids are reasonable.

Whooping cough is a highly contagious disease. It can also be lethal in rare cases, particularly with infants too young to be given the proper vaccination. The disease begins like a cold, but it can progress to severe coughing, which can last for several weeks.

Last year, the hardest-hit state appears to be California. Health officials in the state reported more than 8,300 cases, which includes 10 cases of infant deaths.

At least 26 deaths were reported nationwide, the CDC said at a vaccine advisory committee meeting on Wednesday. However, it is still a preliminary case count and it might end up being higher.

Health officials think that contagious teenagers are a dangerous threat to susceptible infants. Around 95 percent of children already have a minimum of three shots against the disease. However, the whooping cough vaccine for adults and adolescents was not approved until 2005. Thus, they have lower vaccination rates.

The Advisory Committee on Immunization Practices suggest all adults who are around infants to get a whooping cough vaccine. The committee also voted on Wednesday to slightly modify the guidelines on vaccination to ascertain that all health care workers, including nurses, would get the vaccine against the disease.

Adolescents require booster shots to protect them from meningococcal meningitis, a life-threatening infection of the tissue around the brain, while all children should have up to date whooping cough vaccines in line of the recent outbreaks. This is according to new recommendations given by pediatric experts.

The American Academy of Pediatrics releases updated guidelines for vaccinations every year. Its new issue released on Feb.1 in the journal Pediatrics, is very similar to their recommendations from last year.

However, even without major changes, pediatricians said the new issue is a good chance to remind parents to check that their children’s vaccines are up to date.

Immunizations have been the most efficient medical preventive measure ever developed. But right now, some people living in United States do not appreciate how protected  they have been because of vaccines, informed Dr. Michael Brady, chairman of the American Academy of Pediatrics’ committee on infectious disease.

He added that there are still children all over the world that are dying from polio and measles. The schedules of vaccination are designed to get vaccines to the child before they are at risk.

This year’s recommendations include that all children aged 6 months to 18 should get an annual flu shot. Children aged 6 months to 8 years who only had one dose of a previous flu vaccine need two doses of the 2010-2011 seasonal flu vaccine.

Children who didn’t get their pneumococcal vaccinations on time, and are age 5 or under, should get vaccinated with a newer formulation of the vaccine called Prevnar.

All of these and many other recommendations are listed on the AAP guidelines, which were approved by the American Academy of Family Physicians and the CDC’s Advisory Committee on Immunization Practices.