Every year the influenza virus infects millions of people in the US and results in the death of 55,000 people, usually the elderly or sick. To combat this massive global health burden and to prevent pandemics a flu vaccine is manufactured each winter season. The World Health Organization predicts which of the strains of the virus are likely to be most prevalent or problematic and a vaccine targeting three strains is developed. The flu vaccine is single most effective way to prevent infection. To make the vaccines more effective, molecules that stimulate the immune system are added. These are called adjuvants and can include oils or salts. Without these adjuvants, people would require multiple flu vaccines in one season to protect themselves from infection.
Following the H1N1 pandemic in 2009, we found our first glimpse of evidence that people who were obese were at risk for developing more severe complications related to influenza infection. With their increased risk for hospitalization or death, obese people are more dependent on getting the flu vaccine to protect themselves. Unfortunately, there is currently no data on the effectiveness of the flu vaccine in obese individuals. This is a problem since world wide over 1.9 billion adults are overweight and 600 million are obese. To shed some light on the problem of vaccinating obese individuals, a group of researchers based in Tennessee turned to mice and found that obese mice are less protected by influenza vaccines.
To start with, the researchers gave a single dose of a flu vaccine to mice that were normal body weight or were obese and then measured the levels of key indicators of immunity in their blood. The obese mice had around 4 times less immunity against the influenza virus when compared to lean mice and they also had a 5-10 times lower range and magnitude of response to the virus. This means that they were unable to generate as strong a response to a specific virus and were also unable to generate a response to as many different types of the flu virus. In other words, the flu vaccine in these mice would not protect them against other strains of the flu virus as well as it would in lean mice. When the researchers vaccinated the mice and then infected them with the influenza virus, all the unvaccinated lean mice died (after 7 days) while all obese mice died regardless of whether they were vaccinated or not (with or without adjuvant, all after 5 days). This same trend held whether the mice were infected with H1N1 or H7N9. The researchers tried to give the obese mice more doses of the vaccine to confer them a greater resistance and even still all the mice died by 9 days. Finally, to show that it wasn’t a problem with the mice developing antibodies to the vaccine, the researchers took the serum from lean mice given the vaccine and put it into the obese mice given the vaccine. Even in this case the obese mice died following influenza infection. This result tells us that the obese mice could still generate antibodies to the vaccine but that their immune response to those antibodies was deficient and could not eliminate the virus.
These results are very important for our understanding of how the global epidemic of obesity affects the efficacy of influenza vaccines. If obese people are more at risk for influenza related complications then they should get the vaccine to protect them, but if that vaccine doesn’t work for them then what are we supposed to do? More research is clearly needed. We still don’t know why the immune system in these obese mice fail to mount a response to the virus even when vaccinated and we also don’t know if these results hold true in humans. Still, it goes to show how broad of an impact the obesity problem has. It would be frightening if obese people are less protected from all vaccines we use to protect against dangerous diseases.