Each winter, many people wonder whether its worth getting a flu shot while health officials repeatedly warn against not getting one.

Now a new study sheds light on the benefit of doing so. Even when the flu shot is just 20% effective it can still reduce US doctor visits due to illness by an estimated 20 million in a single year, the new report published in the scientific journal PNAS (Proceedings of the National Academy of Sciences of the United States) finds.

In addition, vaccination also can prevent thousands of hospitalizations and deaths, the study authors estimated based on the average vaccination coverage rate in the United States.

“Getting vaccinated against influenza is beneficial to the individual and to the community even when the vaccine is of relatively low efficacy,” said Burton H. Singer, co-author of the study and an adjunct professor for the Emerging Pathogens Institute at University of Florida in Gainesville.

Caused by viruses, flu is a contagious respiratory illness with mild to severe symptoms that can sometimes lead to death. The flu virus evolves rapidly and new viruses circulate in different parts of the world, so each year scientists must reformulate the vaccine. Add to that an imperfect manufacturing process and even a ‘good match’ reformulation may not be as effective as scientists would like.

In fact, the Centers for Disease Control and Prevention estimated just 36% effectiveness for the 2017-18 seasonal vaccine as of February 3. (The season ends in May.)

For the new study, Singer and his colleagues created a mathematical model of flu transmission and vaccination to evaluate how much illness is prevented by even a very low effectiveness flu vaccine. The research team found that at the average rate of US coverage even a poor vaccine would prevent a significant amount of illnesses, hospitalizations and deaths.

For example, at just 43% coverage (the average rate of Americans who received a flu shot for the years 2012 through 2017), a vaccine with just 20% effectiveness could avert more than 20 million infections or illnesses as compared to not getting the vaccine. In addition, 129,000 hospitalizations and 61,000 deaths could be prevented.

Based on the model, if more people got a flu shot, say half of the US population, the same 20% effective flu shot would prevent an additional 3.63 million infections, 21,987 hospitalizations and 8,479 deaths.

“When a vaccine is fully effective on 50% or more of the people who are vaccinated, you need to primarily focus on vaccinating young children,” said Singer. The reason? Children are still building immunity and they pass germs around at school.

“As efficacy of the vaccine decreases, it becomes increasingly important for the elderly to be vaccinated in addition to young children,” said Singer, since the elderly are more likely to develop complications from the flu, such as pneumonia, which can be deadly.

The CDC reported a total of 160 flu-related deaths in children and 30,064 flu-related hospitalizations overall between October 1, 2017 and April 21, 2018. The highest rate of hospitalization occurred among adults 65 years old and older.

Richard Webby, a flu scientist who is part of the World Health Organization’s advisory board and a member of St. Jude Children’s Research Hospital’s Department of Infectious Diseases, said “effectiveness estimates go up and down based on a number of factors including match of vaccine and circulating strain and probably other factors we don’t fully understand.”

“The take home message from the past few seasons is that there is much room for improvement,” said Webby, who was not involved in the research.

Still, more people, particularly the elderly, need to get vaccinated even when the vaccine effectiveness is lower than hoped, Webby said: “This study suggests that even with a less than optimal vaccine there is still much public health benefit that can be achieved if these are used properly and widely.”