IDCM Issue 2: Flu Quick Facts: Information You Need for Discussing Flu Vaccination with Patients

Post Date: 
Natasha Chida, MD, MSPH

Flu causes serious morbidity and mortality.
The annual global influenza (flu) attack rate for adults is 5-10%, with 250,000-500,000 deaths and 3-5 million cases of severe illness.1 For persons with certain chronic diseases (see below), flu related hospitalizations range from 20-1,000 per 100,000 persons.2

Certain groups are at risk for increased morbidity and mortality from flu.

These include: adults aged 65 years and older; pregnant women; residents of chronic care facilities; people with chronic lung, cardiac, neurologic, renal, liver, blood, endocrine, and metabolic diseases; children on long-term aspirin therapy; persons with a BMI of ≥ 40; people with compromised immunity; children younger than 5, and (in the United States [US]) Americans Indians/Alaskan Natives.3 When persons who aren’t at high risk of severe flu get vaccinated, it helps protect vulnerable groups by preventing transmission. This is especially true for healthcare workers, who work closely with these populations.

Flu vaccines can prevent hospitalizations and mortality.

All available flu vaccines contain either no virus or inactivated (i.e., killed) virus except one, which contains a weakened live virus. The weakened live virus vaccine is administered intranasally, while the others are administered as intramuscular injections. The US Centers for Disease Control and Prevention (CDC) website has information on the different kinds of vaccines that are commercially available in the US, and their contraindications.3

  • One meta-analysis in elderly persons found the pooled efficacies of the vaccine to be: 56% for preventing respiratory illness; 50% for preventing hospitalization, and 68% for preventing death.4 In another study, vaccination reduced hospitalization from cardiac disease (19%) cerebrovascular disease (16%), and pneumonia/influenza (32%); all-cause mortality decreased by 48-50%.5
  • In one study of persons with COPD, the incidence of cases in those who got the flu vaccine was 6.8 /100 person-years vs 28.1/100 person-years in those who did not. The vaccine was 76% effective.6 In another study of elderly persons with chronic lung disease, vaccination was associated with fewer hospitalizations (RR of 0.48) and death (OR 0.30).7
  • One study found that administering the vaccine to pregnant women reduced the risk of influenza by 36%, and reduced influenza-like illness syndromes in their infants by 63%.8  Another found the vaccine efficacy rate for pregnant women and their infants to be 50% and 49%, respectively.9 The vaccine also reduces flu, pneumonia, and hospitalizations in persons with diabetes.10-11


Prioritizing who gets vaccinated depends on resources.

The World Health Organization (WHO) recommends vaccination for pregnant women, people older than 65, people with chronic medical problems, children 6 months-5 years of age, and healthcare workers.1 The CDC recommends vaccination for everyone over 6 months of age. The European CDC recommends vaccination for adults and children with any chronic illness, persons older than 65, pregnant women, and healthcare workers.12

Influenza types are diverse and dynamic—particularly type A, which causes pandemics.

There are 3 types of flu: A, B, and C. Types A and B are the clinically significant ones that cause seasonal epidemics in countries north/south of the equator (in some tropical regions flu circulates throughout the year).3 Influenza A has a wide variety of subtypes (or strains) based on the many different combinations of the virus’ 2 surface proteins: hemagglutinin (H) and neuraminidase (N).3 There are 18 different “Hs” and 11 different “Ns.” Additionally, the virus can change due to genetic mutations that occur through “antigenic shift” or “antigenic drift.” Drift occurs slowly over time, when the virus accumulates genetic changes. Shift is much more rapid, and occurs when there is a new “H” or “N.” This happens when certain animals, like pigs, are infected with multiple flus (human, avian, swine) simultaneously, which allows for genes to mix. Antigenic shift can lead to flu pandemics, but drift generally does not.


Since the circulating flu strain changes each year, being vaccinated previously may not provide protection.

As the strain differs annually, a new vaccine must be made each year. The viral strains included in vaccine are selected based on which strains circulated at the end of the previous year. WHO issues recommendations based on the strains that are predicted to circulate, and countries decide what strains to include in production.
The vaccine’s effectiveness depends partly on how well it is matched to the circulating virus of the season. Usually the vaccine and circulating virus are well matched; when this occurs the efficacy of the inactivated vaccine has been found to be 70-90%.12 Efficacy also depends on host immunity. Elderly and immune compromised persons may have lowered response to vaccination, but as they are at higher risk for complications they should get vaccinated (barring contraindications).13 Vaccination is also recommended for persons with HIV (regardless of CD4) and bone marrow/solid organ transplants (6 months after transplant). There are no official recommendations for persons with malignancy, or on chemotherapy or steroids, but flu vaccine has been found to be safe and efficacious in these groups.14


The flu shot is safe; you cannot get flu from it.  

Injectable flu vaccines are made of either inactivated virus or recombinant influenza vaccine; neither contain live virus, so neither are capable of causing flu. The nasal spray vaccine is made of a weakened live flu virus and can cause symptoms of a cold (runny nose, congestion, sore throat and cough). These milder symptoms should not be confused with influenza, which is characterized by high fever and lower respiratory tract symptoms. The nasal vaccine is not capable of causing flu because the weakened virus cannot affect parts of the body warmer than the oropharynx (ie, the lungs).3

The most common reaction to vaccination are redness and soreness at the injection site. On rare occasions people experience low-grade fever and aches due to an immune response. In addition, several randomized controlled trials have compared symptoms in people who get flu vaccines vs. placebo. All of them showed no difference in body ache, fever, cough, runny nose, or sore throat; the only difference was soreness in the injected arm.15-16 Also, most viral syndromes that occur after the flu shot are found to be due to a virus other than flu.3

Severe reactions to the injectable vaccines in adults are rare. The most common are: anaphylaxis, which occurs in 0.7 per one million persons, and Guillain-Barre syndrome (GBS), which may occur in 1-2 per one million persons.17 These reactions occur with much less frequency than severe influenza. In addition, GBS is thought to occur more commonly after influenza infection than vaccination.
With the nasal spray containing live weakened virus, generalized cold symptoms may occur in 59-60 per 100 adult persons, while anaphylaxis may occur in 1 per 500,000 adult persons.17 The flu vaccine is thus safe, provided healthcare workers ensure patients do not have contraindications.

Bottom Line: The flu can cause significant morbidity and mortality, particularly in high-risk groups. Vaccination is an important public health strategy to improve flu outcomes. 


  1. World Health Organization Web site. Influenza Web page. Accessed 2016-02-17.
  2. US Centers for Disease Control and Prevention. Estimates of deaths associated with seasonal influenza --- United States, 1976--2007. Morb Mortal Wkly Rep. 2010;59(33):1057. Accessed 2016-02-17.
  3. US Centers for Disease Control and Prevention Web site. Influenza (Flu) Web page. Accessed 2016-02-17. 
  4. <Gross PA, Hermogenes AW, Sacks HS, Lau J, Levandowski RA. Ann Intern Med. The efficacy of influenza vaccine in elderly persons. A meta-analysis and review of the literature1995:123(7):518-27. PMID: 7661497. Accessed 2016-02-17. 
  5. Nichol KL, Nordin J, Mullooly J, Lask R, Fillbrandt K, Iwane M. Influenza vaccination and reduction in hospitalizations for cardiac disease and stroke among the elderly. N Engl J Med. 2003:348(14)1322-32. PMID: 12672859. Accessed 2016-02-17. 
  6. Wongsurakiat P, Maranetra KN, Wasi C, Kositanont U, Dejsomritrutai W, Charoenratanakul S. Acute respiratory illness in patients with COPD and the effectiveness of influenza vaccination: a randomized controlled study. Chest. 2004:125(6):2011-20.  Accessed 2016-02-17. 
  7. Nichol KL, Baken L, Nelson A, Zaman K, Roy E, Arifeen SE, Rahman M, Raqib R, Wilson E, Omer SB, Shahid NS, Breiman RF, Steinhoff MC. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med. 2008:359(15):1555-64. PMID: 18799552. Accessed 2016-02-17. 
  8. Madhi SA, Cutland CL, Kuwanda L, Weinberg A, Hugo A, Jones S, Adrian PV, van Niekerk N, Treurnicht F, Ortiz JR, Venter M, Violari A, Neuzil KM, Simões EA, Klugman KP, Nunes MC; Maternal Flu Trial (Matflu) Team. Influenza vaccination of pregnant women and protection of their infants. N Engl J Med. 2014:371(10):918-31. PMID: 25184864. Accessed 2016-02-17. 
  9. Lau D, Eurich DT, Majumdar SR, Katz A, Johnson JA. Effectiveness of influenza vaccination in working-age adults with diabetes: a population-based cohort study. Thorax. 2013:68(7)658-63. PMID: 23535212.
  10. Colquhoun AJ, Nicholson KG, Botha JL, Raymond NT. Effectiveness of influenza vaccine in reducing hospital admissions in people with diabetes. Epidemiol Infect. 1997:119(3):335-41. PMID: 9440437.
  11. European Centre for Disease Prevention and Control Web site. Seasonal influenza Web page. Accessed 2016-02-17.
  12. Kunisaki KM, Janoff EN. Influenza in immunosuppressed populations: a review of infection frequency, morbidity, mortality, and vaccine responses. Lancet Infect Dis. 2009: 9(8)493-504. PMID: 19628174.
  13. Shehata MA, Karim NA. Influenza vaccination in cancer patients undergoing systemic therapy. Clin Med Insights Oncol. 2014: 1(8)57-64. PMID: 24855405
  14. Nichol KL, Lind A, Margolis KL, Murdoch M, McFadden R, Hauge M, Magnan S, Drake M. The effectiveness of vaccination against influenza in healthy, working adults. N Engl J Med. 1995:333(14)889-93. PMID: 7666874.
  15. Nichol KL, Margolis KL, Lind A, Murdoch M, McFadden R, Hauge M, Magnan S, Drake M. Side effects associated with influenza vaccination in healthy working adults. A randomized, placebo-controlled trial. Arch Intern Med. 1996:156(14):1546-50. PMID: 8687262.
  16. World Health Organization Web site. Information Sheet: Observed Rate of Vaccine Reactions: Influenza Vaccine. July 2012. Accessed 2016-02-17.


We gratefully acknowledge Dr. Trish Perl, Professor of Medicine, Johns Hopkins Medicine; Senior Epidemiologist, Johns Hopkins Health System, for her thoughtful review of this work.
The Johns Hopkins Center for Clinical Global Health Education is a clinical research, education, and leadership development center in the Division of Infectious Diseases, Department of Medicine, Johns Hopkins University School of Medicine. We conduct clinical research, and we train, support, and empower healthcare providers and researchers working in resource-limited communities who share our commitment to improve health outcomes.