Last season’s flu vaccine was only moderately effective. Experts don’t know why. As for effectiveness of this year’s vaccine, we’ll have to wait and see.

This year’s flu vaccine has already been formulated.

Manufacturers are now busy producing the serum.

Next month, the vaccine should start being available in certain locations.

Will it be effective in preventing next year’s prominent strain of the flu?

Well, that’s anybody’s guess.

“How effective a vaccine is going to be is always a bit of a mystery,” Brendan Flannery, the lead investigator for the U.S. Flu Vaccine Effectiveness Network, told Healthline.

Last season’s flu vaccine wasn’t as effective as hoped, especially for those aged 65 years or older.

Overall, the flu vaccine for the 2016-2017 flu season was 42 percent effective across all age groups.

But that number declined to just 25 percent for those above the age of 65.

The vaccine was more effective in children aged 6 months to 8 years, with an estimated 61 percent effectiveness.

Despite this, 101 children died due to the flu in the 2016-2017 season.

Preliminary data did suggest the vaccine was effective at reducing the number of influenza-related hospitalizations.

In adults, the number of hospitalizations was reduced by 30 percent last flu season and by 37 percent for adults aged over 65.

“A better vaccine for people 65 and older is definitely needed. In fact, better flu vaccines are needed for all of us,” Stephen Morse, professor of epidemiology at the Columbia University Medical Center, told Healthline.

Flannery said experts aren’t quite sure why this past season’s flu vaccine didn’t perform better.

He said scientists at the Centers for Disease Control and Prevention (CDC) correctly predicted the main strain would be type A H3N2.

However, the effectiveness of the vaccine was below the annual average of 48 percent.

Flannery said the H1 and H3 strains of the flu are usually strong every two to three years.

Influenza B strains can be the predominant ailment in other years.

Every year, CDC scientists try to predict what strain will be the most powerful in the upcoming flu season.

“The vaccine is meant to be protective against the virus that is circulating,” he explained.

CDC scientists made their calculations in February and March.

Flannery said they made one small change in the H1N1 component of the vaccine. The other components, including influenza B and H3N2, are unchanged.

The 2017-2018 vaccine is being manufactured this summer.

Flannery said the first installments of the vaccine should be available next month.

CDC officials recommend most people get vaccinated in September or October before flu season kicks into gear in November.

Flannery said children and adults should get vaccinated despite the lower effectiveness rate.

He said the vaccine does boost a person’s immune system and can help prevent someone from getting a severe flu.

He said people who don’t get the flu often and feel they don’t need the vaccine are rolling the dice.

“It’s a bit of a game of chance,” he said.

Flannery added seniors in particular should get flu shots even if last year’s vaccine wasn’t that effective for their age group.

Morse agrees those aged 65 and over should still get the vaccine because they are most at risk for more severe flu and complications.

“Even 25 percent effectiveness is far better than no vaccine at all. It can still save lives … Flu spreads like wildfire through nursing homes, for example, and even partial protection can dampen the spread,” he said.

Dr. William Schaffner is a professor of medicine in the Division of Infectious Diseases at the Vanderbilt University School of Medicine.

He said that although the flu vaccine isn’t perfect, there are options for a stronger vaccine for those in the older age group.

“There now are three influenza vaccines that perform better than the standard vaccine in this older age group. The high-dose vaccine for which there is the most extensive supporting data as well as the adjuvanted vaccine and the recombinant vaccine,” he told Healthline.

“[The] flu can knock you down, and older, frail persons may not ever completely recover. Their capacity to function in activities of daily living may be permanently diminished. Thus, every ounce of prevention is vital,” Schaffner said.

Schaffer said this is also true for children, noting the 101 youngsters who died from this season’s flu.

“This is profoundly sad. Although the data for this year are not yet in, if previous years are a guide, the substantial majority of these children will turn out not to have been vaccinated and many were completely healthy children,” Schaffner said. “Here in the U.S., about 50 to 60 percent of children receive flu vaccine each year — that still leaves many unprotected.”

Estimates from the CDC of early season vaccination coverage in the 2016-2017 flu season show that only two out of five people aged 6 months and older had received a flu vaccine by November 2016.

Schaffner said more people need to get vaccinated.

“Influenza is a very serious infection. Here in the U.S., it causes an average of over 200,000 hospitalizations each year, and, depending on the severity of the flu season, from 4,000 to 40,000 excess deaths. Even young, healthy people can get a serious flu illness that puts them in the intensive care unit,” he said.

The biggest benefit of vaccines, said Morse, is the notion of “herd immunity.”

An infection can be stopped if there are enough people who are resistant to it. The level of “herd immunity” needed depends on how transmissible the infection is.

“Surprisingly, for flu, herd immunity requires a bit over 50 percent to work,” Morse said. “If everyone had the vaccine, and it was only 50 percent effective, we’d almost be at that level. Other measures, such as ‘social distancing’ and staying home when sick, could make up the remainder. Doesn’t seem that hard to achieve, but we’ve never been able to do it.”

The CDC’s Advisory Committee on Immunization Practices has recommended that FluMist, a nasal spray that delivers the influenza vaccine, not be used in the 2017-2018 season.

Both Morse and Schaffner emphasized that regardless of the efficacy of the vaccine, it only works if you take it, and it could save lives.

“Flu vaccine is not perfect and we all wish that it were more effective, but it remains our best current preventive measure,” Schaffner said.