Background
Adults with cancer are prone to serious complications from influenza, more than healthy adults. The influenza vaccine protects against influenza and its complications. However, its effectiveness among people with cancer is unclear, as immune dysfunction that accompanies cancer as a result of chemotherapy might lower immune response to the vaccine. People with cancer, therefore, do not have clear information on the importance and effectiveness of the vaccine.

The aim of the review
This review focused on the effectiveness of influenza vaccination in adults with cancer who have a suppressed immune system because of cancer or chemotherapy. We searched the literature up to May 2017.

What are the main findings?
We identified six clinical studies (2275 participants) addressing this question, half of which were randomised controlled trials, where patients were randomly selected to get or not to get the vaccine. Two other studies showed that adults with cancer who were vaccinated were found to have lower rates of death, but these studies were not randomised. One small randomised study showed a similar mortality rate in vaccinated and non-vaccinated patients. Pooling (combining) results from the different studies was not possible because of different methods or different ways the results were reported. There was a lower rate of influenza-like illness (any febrile respiratory illness), pneumonia, confirmed influenza and hospitalisation for any reason, among vaccinated people in at least one study. No side effects to the vaccine were reported in these studies. The review also included a trial comparing the regular vaccine to a vaccine containing an adjuvant that is supposed to increase the immune response. This randomised trial was small and did not find differences in all clinical outcomes examined.

Quality of the evidence
The strength of evidence is limited by the low number of studies and by their low methodological quality (high risk of bias).

What are the conclusions?
It is unlikely that there will be any future large-scale controlled trials to investigate this issue. The current evidence, although weak, suggests a benefit for influenza vaccination amongst adults with cancer and the vaccine was not found to be harmful. Influenza vaccines given to adults with cancer contain an inactivated virus that cannot cause influenza or other viral infection.