Last year, fewer people were vaccinated against influenza, while the flu season is the deadliest in decades.

According to a new report published by the Centers for Disease Control and Prevention, about four in ten adults in the United States were vaccinated against influenza last year, less than last year and previous years.

The new analysis, which encouraged physicians to recommend and offer influenza vaccines, provides a more complete picture of the 2017-2018 influenza season, the deadliest of the last four decades.

The study found that vaccination rates were consistent among adults of all ages, with vaccination coverage among all age groups of adults, reaching the lowest levels in seven influenza seasons.

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According to the CDC, approximately 37% of US adults received influenza vaccine during the 2017-2018 influenza season, a decrease of about 6% from the previous year.

This year's flu season has already begun. The CDC recommends that all persons six months of age and older be vaccinated by the end of October; According to the new study, most adults tend to be vaccinated against the flu before the end of November.

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Influenza vaccines protect against disease by stimulating the creation of protective antibodies in the body. They must be administered every year and are available everywhere, from doctors' offices to pharmacies, health centers and even many offices.

According to the CDC, it usually takes about two weeks after the vaccination for the antibodies to develop.

Influenza varies from season to season in terms of severity and the most common type of influenza virus. Influenza vaccines are designed with this in mind; even in this case, efficacy may vary, in part because manufacturers often need to determine which viruses are likely to dominate the flu season of the year by January.

The new CDC report estimated influenza immunization coverage using its Behavioral Risk Factor Surveillance System, which uses telephone surveys to track health data across the 50 states.

The study had some major limitations, including respondent-reported immunization status and lower response rates.

Notably, other government data sources – including other surveys and data on Medicare claims – do not appear to show a decline in influenza vaccine coverage, the study said.