While this answer briefly describes the most frequently observed side effects of the most common flu shots, it goes into a bit more detail to explain the circumstances that make it difficult to ascertain the extent to which flu shots are beneficial.
Since the inactive flu shot is a mix of surface flu antigens and not a live virus, the shot itself cannot give flu, and local injection site reactions and cold-like symptoms are its most commonly reported side effects ().
Not recommended for those younger than 2 or older than 50 years nor for pregnant women nor those taking salicylate treatment nor the immunodeficient nor immunosuppressed, theis a far less-used alternative that’s also for as-yet unknown reasons found to be less effective ( ). Person-to-person transmission of LAIV isn’t a known risk ( ) while most frequently observed side effects (runny nose, congestion) aren’t known to to increase flu risk either.
In short, neither the inactivated flu shot nor LAIV are known to increase the risk for contracting flu.
Why It’s Challenging to Assess Flu shot Effectiveness
The main components in the inactivated flu shot are its surface antigens,(HA) and (NA).
Flu virus can reassort its surface antigens fairly rapidly to generate new strains every few years (see below from 4),
‘New antigenic variants of A/H3N2 viruses appear every 3–5 years, whereas new antigenic variants of A/H1N1 and influenza B viruses appear less frequently (2–5 years for A/H3N2 viruses compared with 3–8 years for A/H1N1 and influenza B viruses).’
Flu’s ability to reassort to create new strains requires a vaccine capable of protecting across subtypes and no such vaccine currently exists.
It isn’t presently possible to proffer an ironclad guarantee that each person getting a flu shot would be fully protected against whatever flu’s going around since current flu vaccines tend to offer variable and often sub-optimal degree of protection, which has to do with two main drawbacks.
- Gaps in knowledge on what it takes to drive robust, sustained, effective immune responses across flu subtypes stymie the development of a universal flu vaccine.
- Using an early snapshot of circulating flu strains as the basis for choosing the flu vaccine in a given season.
Based on a few pre-season snapshots of flu strains circulating prior to a given flu season, each year the WHO makes predictions about the major flu strains likely to circulate that year. Temperate region countries then make a new seasonal flu vaccine consisting of HA and NA antigens derived from the early circulating flu strains (flu circulates year-round in tropical regions).
Since flu strains that circulate later can be different from those found circulating earlier, seasonal flu vaccines end up the product of a scattershot approach as likely to hit as miss, which is why depending on how well the pre-season predictions end up matching the flu strains that actually circulate, seasonal flu vaccine effectiveness can vary greatly from year to year as the Wikipedia page shows,.
Since the flu shot changes each year, gauging its effectiveness is tricky and indirect, involving ambiguous measures such as reported rates of flu-like illness, days of work or daycare missed by adults and children, respectively, etc. Obviously, such vague and indirect measures yield a far from accurate assessment of vaccine effectiveness.
Such inaccuracy is compounded by the fact that age, health status and local factors in terms of intensity of transmission and predominant circulating strains play an outsize role in influencing effectiveness of a given season’s flu shot.
Finally, though the flu shot value may be more evident at the population level in terms ofthan at the individual level, that isn’t possible to assert conclusively either since the quality of existing studies tend to be poor (biased methodology, conduct , measures). For example, a comprehensive 2010 Cochrane review that examined studies where healthcare workers working in long-term care facilities got flu shots concluded (see below from )
‘there is no evidence that vaccinating HCWs prevents influenza in elderly residents in LTCFs.’
1. Demicheli, Vittorio, et al. “Vaccines for preventing influenza in healthy adults.” The Cochrane Library (2014).
2. Caspard, Herve, et al. “Live-attenuated influenza vaccine effectiveness in children from 2009 to 2015–2016: a systematic review and meta-analysis.” Open forum infectious diseases. Vol. 4. No. 3. Oxford University Press, 2017.
3. Izurieta, Hector S., et al. “Adverse events reported following live, cold-adapted, intranasal influenza vaccine.” Jama 294.21 (2005): 2720-2725.
4. Petrova, Velislava N., and Colin A. Russell. “The evolution of seasonal influenza viruses.” Nature Reviews Microbiology 16.1 (2018): 47.
5. Thomas, Roger E., Tom Jefferson, and Toby J. Lasserson. “Influenza vaccination for healthcare workers who work with the elderly.” Cochrane Database Syst Rev 2 (2010).