Pulmonology Pulmonology
Pulmonol 2018;24:260-2 - Vol. 24 Num.4 DOI: 10.1016/j.pulmoe.2018.05.004
Letter to the Editor
Moderate influenza vaccine effectiveness in a B mismatch season: Preliminary results from the 2017/2018 season in Portugal
A. Machadoa,b,, , I. Kislayaa,b, B. Nunesa,b, A.P. Rodriguesa, R. Guiomarc, On behalf of EuroEVA project
a Department of Epidemiology, National Health Institute Dr. Ricardo Jorge, Lisbon, Portugal
b Centro de Investigação em Saúde Pública, Escola Nacional de Saúde Pública, Universidade NOVA de Lisboa, Lisboa, Portugal
c Department of Infectious Diseases, National Health Institute Dr Ricardo Jorge, Lisbon, Portugal
Background and objectives

Vaccination has become one of the main public health measure for influenza control, its role in reducing the risk of developing the disease and post-infection complications1 is widely recognised. Influenza vaccination is recommended for the elderly, for individuals with chronic conditions, the pregnant and health professionals. Due to the continuous changes of the influenza virus, the seasonal flu vaccine composition is reformulated annually, to match viruses that are expected to circulate in that season. Early in the season influenza vaccine effectiveness (IVE) estimate is an important indicator of the need for further public health measures, it is critical when the IVE is low.

This study aimed to provide early 2017/2018 seasonal IVE using data from the Portuguese influenza surveillance system.


This analysis included patients with influenza-like illness (ILI) from primary care and emergency departments swabbed for the detection of influenza using RT-PCR and reported to the National Influenza Surveillance System (NISS)2 between weeks 38/2017 to 5/2018. We used the test-negative case–control study design,3 where influenza laboratory confirmed incident ILI patients (Cases) were compared to laboratory influenza negative ILI patients (Controls). A medical doctor collected demographic and clinical data, ILI signs and symptoms, onset and swab collection dates and vaccination status during the consultation or through the clinical record. An ILI patient was considered vaccinated if the 2017/2018 influenza vaccine uptake occurred 14 days before symptom onset. IVE analysis was restricted to ILI patients with symptoms compatible with the European Union ILI definition,4 i.e., with sudden onset of at least one systemic and one respiratory symptom. Chi-square test was used to compare baseline characteristics between Cases and Controls. Crude IVE, design adjusted for calendar time, was estimated using 1-odds ratio (OR) of being vaccinated in Cases vs. Controls and was further adjusted for confounding by age group and presence of chronic condition.


A total of 732 ILI patients were reported to the NISS and approximately 74% adhered to the ILI definition (Table 1).

Table 1.

Description of notified ILI patients and comparison of Cases and Controls according to sex, age group, presence of chronic condition with indication to vaccine uptake, setting, influenza vaccine uptake and ILI clinical definition.

  ILI patientsControlsCasesp-Value* 
  n  n  n   
Female  402  55.2  132  56.2  135  56.3   
Male  326  44.8  103  43.8  105  43.8   
Total  728    235    240     
Age group0.001 
<18  115  16.1  24  10.4  52  22.3   
18–44  256  35.9  85  36.8  84  36.1   
45–64  213  29.9  71  30.7  69  29.6   
65+  129  18.1  51  22.1  28  12.0   
Total  713    231    233     
Chronic illness0.027 
No  487  66.5  148  62.7  174  72.2   
Yes  245  33.5  88  37.3  67  27.8   
Total  732    236    241     
Primary carea  299  40.9  125  52.9  115  47.7   
Emergency roomsb  433  59.2  111  47.0  126  52.3   
Total  732    236    241     
Influenza vacination
No  548  84.6  158  74.5  200  90.9  <0.001 
Yes  100  15.4  54  25.5  22  9.9   
Total  648    212    222     
ILI EU definition
No  188  25.7           
Yes  544  74.3           
Total  732             

Includes general practitioners from the Rede Médicos Sentinela.


Includes emergency rooms from hospitals and obstetrician wards.


p-Value of chi-square test for comparison of Cases vs. Controls.

Statistical significant differences were verified in the age group distribution between Cases and Controls (Table 1). Cases presented a lower proportion of at least one chronic condition and vaccine coverage.

Using data updated up to week 5 (Table 2), adjusted IVE was estimated in 59.6% (95%CI: 22.1–78.4%). Considering the weekly IVE monitoring, results observed at the end of the period were very similar to the ones obtained at the beginning of the epidemic period (week 2) (even though with low precision). This result highlights the potential of having very early indicative estimates using surveillance data.

Table 2.

Crude and adjusted IVE against medically attended influenza, week 1 to week 5.

  Virus subtype or lineage  % (nEV crudea (%) (95%CI)  EV adjustedb (%) (95%CI) 
Week 1A(H1)pdm09  12.6 (13)  66.3%
25.2% to 84.8%
-5.2% to 82.7%
A(H3)  2.9 (3) 
B/Vic  13.6 (14) 
B/Yam  41.8 (43) 
B lineage not ascribed  29.1 (30) 
Week 2A(H1)pdm09  13.8 (20)  69.0%
37.7% to 84.6%
13.9% to 82.9%.
A(H3)  4.8 (7) 
B/Vic  12.4 (18) 
B/Yam  49.0 (71) 
B lineage not ascribed  20.0 (29) 
Week 3A(H1)pdm09  13.2 (22)  59.1%
23.5% to 78.1%
-7.0% to 74.9%.
A(H3)  4.2 (7) 
B/Vic  13.2 (22) 
B/Yam  57.5 (96) 
B lineage not ascribed  12.0 (20) 
Week 4A(H1)pdm09  15.3 (33)  64.9%
38.1% to 80.2%
17.0% to 77.4%
A(H3)  5.1 (11) 
B/Vic  11.6 (25) 
B/Yam  56.5 (122) 
B lineage not ascribed  11.6 (25) 
Week 5A(H1)pdm09  15.8 (38)  68.6%
45.8% to 81.7%
22.1% to 78.4%
A(H3)  6.6 (16) 
B/Vic  10.8 (26) 
B/Yam  61.0 (147) 
5.8 (14) 

Adjusted for calendar time.


Adjusted for calendar time, age group (0–64; 65 and more years) and presence of at least one chronic condition with indication for the vaccine uptake (chronic respiratory, cardiovascular, renal or kidney condition; diabetes; immunocompromising condition).


Adjusted for calendar time modelled by restricted cubic spline with 3 knots, age group (<18, 18–44, 45–64, 65 and more years), setting and presence of at least one chronic condition with indication for the vaccine uptake.


The 2017/2018 season in Portugal has been characterised by A(H1)pdm09, AH3 and B viruses co-circulation, with predominance of B/Yamagata lineage. Considering the composition of the seasonal trivalent vaccine, which contains B/Victoria component, this constitutes a lineage mismatch which potential affects IVE.

However, our results indicate that the 2017/2018 seasonal vaccine conferred moderate protection against medically attended influenza. Given that the majority of cases were positive for B/Yamagata (61%), this could reflect some cross-lineage protection. Similar results were already reported in Canada, where IVE against B cases was estimated in 55% (95% CI: 38–68%), also in a context of B/Yamagata virus dominance.5 The results obtained this season are also in line with meta-analysis of IVE by virus subtype.6

Our results were estimated using surveillance data, not collected from IVE estimates, with missing information on some relevant factors. For this reason, from the initial sample of ILI patients, 74% were included in the analysis. In addition, this limited the number of variables that could be included in the model as confounders, particularly in earlier weeks due to low sample size. Nevertheless, and besides this limitation, when comparing week 2 with week 5 estimates (which were adjusted for more variables), IVE point estimates remained stable.


In a season with B/Yamagata dominance and with vaccine lineage mismatch, the 2017/2018 trivalent seasonal influenza vaccine conferred moderate protection against medically attended influenza.

The use of surveillance data constituted a useful tool to have early in the season IVE estimates.7 These results assist modifications to health interventions, such as using antiviral treatment in high-risk patients, reinforcement of social eviction and individual hygiene measures to reduce risk of influenza transmission, regardless of the vaccination status.

Conflicts of interest

The authors have no conflict to declare.

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EuroEVA project: Verónica Gómez, Department of Epidemiology, National Health Institute Dr. Ricardo Jorge, Lisbon, Portugal. Inês Costa, Department of Infectious Diseases, National Health Institute Dr Ricardo Jorge, Lisbon, Portugal. Paula Cristóvão, Department of Infectious Diseases, National Health Institute Dr Ricardo Jorge, Lisbon, Portugal. Pedro Pechirra, Department of Infectious Diseases, National Health Institute Dr Ricardo Jorge, Lisbon, Portugal.

Corresponding author. (A. Machado ausenda.machado@insa.min-saude.pt)
Copyright © 2018. Sociedade Portuguesa de Pneumologia
Pulmonol 2018;24:260-2 - Vol. 24 Num.4 DOI: 10.1016/j.pulmoe.2018.05.004
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