Can the flu shot give you the flu?
Here is what the checked sources say. Each title links to the original.
Influenza: The Disease & Vaccines
CHOP says the virus in the flu shot is inactivated, so it cannot cause an influenza infection; only the nasal spray contains live, weakened virus, which can cause a runny nose or low fever. CHOP lists shot side effects as pain, redness and swelling where it was given, plus fever or muscle aches, which it says may occur in people who never had flu or the vaccine before. Guillain-Barré syndrome can very rarely follow vaccination; CHOP says GBS rates are about 17 times higher after flu infection than after vaccination, without giving a time window.
Vaccines for preventing influenza in healthy adults
This Cochrane review of 52 trials with over 80,000 healthy adults compared flu shots with placebo or no vaccine. The shot did cause some side effects: fever rose from 1.5% in unvaccinated people to 2.3% in vaccinated people, so fever can be a real vaccine reaction. The reviewers found no evidence of an association between flu vaccination and serious adverse events in the studies they examined. Earlier versions also used observational studies to look for serious and rare harms; those searches are no longer updated. One author disclosed past consulting for GSK and a Sanofi trial committee role.
Association between Guillain-Barré syndrome and influenza A (H1N1) 2009 monovalent inactivated vaccines in the USA: a meta-analysis
Pooling six US safety monitoring systems covering about 23 million people who got the 2009 H1N1 pandemic flu vaccine, researchers found a small increased risk of Guillain-Barré syndrome, a rare nerve disorder that can cause weakness or paralysis. In a risk window after vaccination, compared with the same people at other times, the rate was about 2.35 times higher, about 1.6 extra cases per million vaccinated. The authors said this matched earlier seasonal-vaccine estimates and that, given the harm caused by 2009 H1N1 flu, the benefits of the pandemic vaccine greatly outweighed the risk. Funded by the US federal government.