World Journal of Cardiovascular Diseases, 2013, 3, 476-482 WJCD
http://dx.doi.org/10.4236/wjcd.2013.37075 Published Online October 2013 (http://www.scirp.org/journal/wjcd/)
Seroprevalence study of infection with influenza
A(H1N1)pdm09 virus in San Felipe Town, Chile
Olea Andrea1, Fasce Rodrigo2, Aguilera Ximena3, Oliva Otavio4, Muñoz Sergio4, García Maritza1,
Pedroni Elena4, Nájera Manuel3, Guerrero Andrea1, Bustos Patricia2, Mora Judith2, Vega Jeanette3,
Cerda Jaime5, Abarca Katia5, McCarron Meg6, Widdowson Marc-Alain6, Castillo Ernesto7,
Alvarado Sergio7, Quijada Sandra7, Gallegos Doris1, González Claudia8
1Center of Epidemiology and Health Policies Department, Faculty of Medicine, Universidad del Desarrollo, Concepción, Chile
2Section of Respiratory and Exanthematous viruses, Sub-Department of Clinical Virology, Institute of Public Health, Ministry of
Health, Santiago, Chile
3Center of Epidemiology and Health Policies of the Universidad del Desarrollo, Concepción, Chile
4Panamerican Health Organization, Washington DC, USA
5Public Health Department and Pediatrics Division, Faculty of Medicine, Pontifical Catholic University of Chile, Santiago, Chile
6Centers for Diseases Control and Prevention, Atlanta, USA
7Center for Micro Data, Economics Department, University of Chile, Santiago, Chile
8Epidemiology and Health Policies Department, Universidad del Desarrollo, Concepción, Chile
Received 14 August 2013; revised 16 September 2013; accepted 1 October 2013
Copyright © 2013 Olea Andrea et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Objective: To know the natural history of the first
wave of pandemic influenza A(H1N1)pdm09 in the
Southern hemisphere, through the detection of anti-
bodies against influenza A(H1N1)pdm09 in a selected
community, to estimate the population attack rate
and its variations, the consultation rates, hospitalliza-
tion and mortality rates. Methodology: A representa-
tive random sample of the population of a commune
in Chile (San Felipe) was interviewed and taken blood
samples between January and March 2010. A study
against the antibodies of the influenza A(H1N1) pdm09
virus was conducted, by the technique of the Hema-
glutination Inhibition (HAI) according to standardiz-
ed methodology. Subjects with antibody titers ≥1:40
were considered positive. Results: 13.5% of the popu-
lation of San Felipe had antibodies against influenza
A(H1N1)pdm09; this percentage reached 30% of the
population between 0 and 18 years and 6.1% among
those over 19 years. The age variable was the only
factor that evidenced significant differences in the pre-
valence of antibodies. There were no significant diffe-
rences related to gender, vaccination history against
seasonal inluenza, or comorbidity. 51% of people
with positive serology showed IN-FLUENZA-LIKE
SYMPTOMS. Conclusions: A relevant percentage of
subclinical disease was detected in the first pandemic
wave in Chile and the proportion of people with SARI
and deaths was small. Data from epidemiological sur-
veillance were useful to estimate the trend of TSI but
not its magnitude.
Keywords: Influenza A(H1N1)pdm09; Influenza
Seroprevalence
1. INTRODUCTION
Chile, along with other countries in the temperate zone
of the southern hemisphere, was one of the first countries
strongly affected by the influenza A(H1N1)pdm09 virus
at the beginning of the pandemic wave. The first case
was confirmed on May 17, 2009 and until December of
the same year, 368,118 cases of influenza like illness(ILI)
were notified in the whole country; 12,302 cases of spe-
cific PCR in real time were confirmed and 634,450 peo-
ple from both the public and private health system were
delivered for free antiviral treatments. During the same
period, 1622 cases of severe acute respiratory infection
(SARI) due to H1N1 (2009) and 150 deaths (deaths re-
lated to pandemic influenza) were confirmed. Among the
cases of ILI, the highest rate corresponded to the 5 - 14
year-old layer, followed by the <5 year-old layer; the ≥60
year-old layer showed the lowest rate. The pandemic
influenza activity peaked between the epidemiological
weeks (EW) 26 and 28, and then it declined and remain-

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 477
ed low from the week 30. 99% of the influenza A viruses
that circulating in Chile during 2009 corresponded to the
influenza A H1N1(2009) [1] virus.
In this context and considering the need to know the
natural history of the first wave of pandemic H1N1(2009)
influenza in the Southern hemisphere, the Ministry of
Health in Chile through its Department of Epidemiology
and the Panamerican Health Organization and the Cen-
ters for Disease Control and Prevention in Atlanta car-
ried out a seroprevalence study at the end of the first pan-
demic wave. The aim was to detect antibodies against in-
fluenza A (H1N1) 2009 in a selected community, in order
to estimate the population attack rate and its variations,
the consultation rates, hospitalization and mortality rates,
while describing the control measures implemented.
The study was designed and funded jointly by the three
entities involved (Epidemiology-MINSAL-Chile, PAHO,
CDC in Atlanta), including the payment of extra human
resources, their training, the installation of a laboratory
for RT-PCR in San Felipe town and the purchase of the
necessary laboratory supplies for IFI and PCR-RT testing.
The sample design was commissioned to the Center for
Microdata of the Economics Faculty of the University of
Chile; the gathering of in-situ information and samples
was carried out by the Pediatrics Division and the De-
partment of Public Health of the Pontifical Catholic Uni-
versity of Chile. The laboratory analysis was carried out
by the Sub-Department of Virology at the Public Health
Institute of Chile.
2. MATERIAL AND METHODS
The design corresponds to a cross-sectional analytical
study by administrating a serological survey to a random
and representative sample of the population of the San
Felipe commune to measure the infection prevalence
with the Influenza A(H1N1)pdm09 virus at the end of the
first pandemic wave (January 2010). In order to carry out
the study, San Felipe commune which is located 94 km
northern Santiago in Chile (32˚45' South latitude 70˚43'
West longitude) was chosen for having easy ways of ac-
cess from Santiago, the capital city, and for having a
small population and the 78% of them gets medical at-
tention from the public health facilities in town. San Fe-
lipe has an area of 186 km2 and the population is of
75.412 inhabitants, 92% urban (estimate in 2009 by the
NIS1).The health infrastructure includes a base 226-bed
hospital, an Emergency Primary Health Care Service,
two urban General Practices Centers and a rural Primary
Care Center. The Commune does not have private hospi-
tals, only outpatient facilities of the Chilean Safety Asso-
ciation and of the Institute of Work Safety.
Sample-Size Calculation. A stratified random sam-
pling by age groups (0 - 4 years old, 5 - 18 years old and
19+ years old) was carried out, using the consumption of
antiviral in the commune as the estimate of prevalence
(attack rate); that is, given that in Chile free antiviral
treatment was delivered to all people who suffered from
a TSI and that the main of the influenza viruses circulat-
ing in 2009 was the influenza A(H1N1)pdm09 virus, it is
possible to assume that the number of treatments pro-
vided are a good approximation to the number of the
people who got ill. For the 0 - 4 years old and 19+ years
old strata, it was considered a 2% attack rate, a 1.5%
absolute error and a 95% confidence level, while for the
stratum 5 - 18 years old, it was considered a 6% attack
rate, a 2% absolute error and a 95% confidence level.
The initial calculation of the sample size was amplified
by 2.0 (design effect), reaching a sample size of 2.350
subjects, to which it was added 30% on account of over-
sampling (3.055 subjects) (Table 1).
0 - 4 year-old and 5 - 18 year-old stratum were
over-represented while the 19+ was underrepresented in
the sample. By expanding the sample, its composition by
age was equal to the population of the commune of San
Felipe.
Multistage Design and Registration. To ensure rep-
resentative population, subjects were selected based on
multistage random sampling; blocks were the primary
sample units, dwellings were the secondary and people
the tertiary. 3.077 dwellings were registered which pro-
duced a list of addresses to visit and subjects to contact
during the field work.
2.1. The Instrument
A 35 question questionnaire was designed with informa-
tion about demographic aspects identification, birth date,
marital status, welfare, educational level, employment),
clinical aspects (morbid history, pregnancy) and epidemi-
ological information (influenza-like symptoms between
May and September 2009, medical consultation, test tak-
ing, hospitalization, antiviral therapy, 2009 seasonal anti-
influenza vaccination history, people ill at home).
2.2. Ethical Aspects
The study was approved by an ad-hoc ethics committee
constituted to review all the studies related to the 2009
Pandemic Influenza and by the Ethics Committee of the
Pontifical Catholic University of Chile; it counted with
an informed consent that had to be signed by everyone
over 18 years old and the parents (or legal tutors) of mi-
nors, as well as the consent of children and adolescents
etween 7 and 17 years old. b
1National Institute of Statistics.

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 478
Table 1. Population of the San Felipe commune, sample size and subjects surveyed with blood samples according to age strata.
Subjects surveyed Subjects with blood samples
Age Population (a) Sample
n % (b) n % (c)
0 - 4 5.848 634 527 83.1 393 74.6
5 - 18 17.567 1050 1.111 105.8 1.028 92.5
19+ 52.541 666 812 121.9 769 94.7
Total 75.956 2.350 2.450 104.3 2.190 89.4
(a) Population estimate (Linear interpolation value, Dic. 2010), Deff: design effect (2,0). (b) The denominator corresponds to the sample size of the correspond-
ing stratum. (c) The denominator corresponds to the surveyed of the corresponding stratum.
2.3. Funding
The study was funded by the Ministry of Health, the Pa-
namerican Health Organization and the Centers for Dis-
ease Control and Prevention (CDC), Atlanta, United States
of America.
2.4. Field Work and Sampling
The field work was carried out in 8 weeks, from January
to the first fortnight of March 2010. The samplings were
taken by university-educated nurses through venous punc-
ture in the crook of the elbow, collecting 10 ml (adults)
or 5 ml (children under 30 kg). The samples were labeled
and transported in cold chain to the commune hospital.
The questionnaires were administered once the samples
were taken.
2.5. Procedures and Laboratory Analysis
In the laboratory of the San Camilo Hospital of San Fe-
lipe, the samples were centrifuged to remove blood se-
rum which was stored at −20˚C. Weekly, the samples
were transferred to the Respiratory Viruses and Exanthe-
matous Section of the Subdepartment of Clinical Virol-
ogy of the Public Health Institute (PHI) where the study
of the antibodies against the A(H1N1)pdm09 virus, by
the technique of the hemagglutination inhibition (HI) ac-
cording to standarized methodology [2].
The sera were previously treated with receptor des-
troying enzyme (RDE, vibrio cholerae Neuraminidase,
Denka Seiken Corporation, Ltd., Tokio, Japón), incubat-
ed at 37˚C to inactivate nonspecific inhibitors of hemag-
glutination. The samples were then incubated at 56˚C for
30 minutes to inactivate the RDE.
As an antigen, a strain of the pandemic influenza A
(H1N1) virus was used, the A/Santiago/34116/09 (H1N1)
20009, isolated from a patient in embryonated chicken
eggs at the PHI an confirmed by RT-PCR in real time,
using the primers and the recommended protocol by the
CDC [3].
The hemagglutination was carried out with turkey ery-
throcytes at a concentration of 0.5%. Controls provided
by the CDC were used: a control antigen strain 2009 A
(H1N1) (A/California/7/2009) and a reference antiserum
for 2009 A (H1N1).
The assay was performed in duplicate for each of the
samples from an initial dilution of 1:10 to a final dilution
of 1:1.280.
The results were recorded as titers or geometric medi-
ans corresponding to the reciprocal of the highest posi-
tive dilution.
In the present study, the proportion of subjects with
antibody titers ≥1:40 were considered positive and term-
ed seroprevalence.
2.6. Statistical Analysis
The statistical analysis was carried out using the module
“Complex Samples” of the SPSS software, version 15.0.
Two types of expansion factors were calculated: one for
the subjects who completed the questionnaire and one for
the subjects who provided blood samples. Not all the
subjects who answered the questionnaire provided blood
samples (Table 1).
The overall and specific antibodies prevalence as well
as the clinical attack rate2, were calculated according to
the age strata3. Those subjects who reported influenza
like illness (fever above 38˚C with coryza or cough) be-
tween May 1 and September 1, 2009 were considered
clinical cases. The subjects were classified into four stag-
es according to the associated antibodies and the symp-
tomatic illness: symptomatic seropositive, asymptomatic
seropositive, symptomatic seronegative and asymptoma-
tic seronegative. Odds ratios were calculated according
to age strata (0 - 4, 5 - 18 y 19+ years old), sex, medical
welfare (Fonasa, public sector/Isapre, private sector),
2009 seasonal anti influenza vaccine history and comor-
bidities (0, 1, ≥2 morbid history, as cancer, diabetes, as-
thma, heart diseases, COPD, hypertension, smoking,
obesity, HIV/AIDS). A logistic regression model (EN-
TER modality) was used for the adjustment of odds ra-
tios.
2Clinical attack rate = n˚of subjects with positive report of influenza
like illness/total n˚ total of surveyed subjects *100.
3Global and specific prevalence of antibodies = n˚ of subjects with
p
ositive blood serum/total n˚ of blood sample donors *100.

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 479
In the same way, the rates of use of antiviral drugs,
consultations and hospitalizations were calculated [4] in
order to describe the access of the population to health
services.
Based on the anti influenza A H1N1-2009 antibodies
prevalence and the influenza like symptoms prevalence
found in San Felipe, the theoretical number of subjects
with anti influenza A H1N1-2009 positive serum for the
total Chilean population [4] was estimated.
3. RESULTS
3.077 subjects were registered, 2.919 visits were made
completing 2.450 questionnaires. This figure exceeded in
4.3% the estimated sample size (Table 1); 2.190 subjects
(89%) provided blood samples, being the 0 - 4 year-old
stratum the smaller group (74.6%).
Table 2 shows the main demographic, clinical and epi-
demiological characteristics of the 2.450 subjects survey-
ed, according to age strata.
The population prevalence of antibodies against influ-
enza 2009 A (H1N1) was 13.5% (CI 95%; 11.3 - 15.9),
reaching about 30% in the 0 - 4 and 5 - 18 year-old strata,
while in those 19+ only reached 6.1%. Age was the only
variable that showed significant differences in the preva-
lence of antibodies (Table 3).
Although the antibodies prevalence was higher among
Table 2. Description of the 2.450 subjects surveyed, according to age stratum.
Subjects surveyed (%)
Age (years) 0 - 4 (n = 527) 5 - 18 (n = 1.111) 19+ (n = 812) Total (n = 2.450)
Gender
Female 270 (51.2) 573 (51.6) 486 (59.9) 1.329 (54.2)
Male 256 (48.6) 537 (48.3) 326 (40.1) 1.119 (45.7)
Welfarea
Fonasa 464 (88.0) 949 (85.4) 651 (80.2) 2.064 (84.2)
Isapre 30 (5.7) 70 (6.3) 50 (6.2) 150 (6.1)
Other
Home inhabitantsb
4 or less 238 (45.2) 654 (58.9) 594 (73.2) 1.486 (60.7)
5 or more 270 (51.2) 445 (40.1) 173 (21.3) 888 (36.2)
Comorbilityc
0 461 (87.5) 920 (82.8) 384 (47.3) 1.765 (72.0)
1 39 (7.4) 157 (14.1) 274 (33.7) 470 (19.2)
2 or more − 11 (1.0) 148 (18.2) 159 (6.5)
Vaccine 2009d
Yes 211 (40.0) 153 (13.8) 289 (35.6) 653 (26.7)
No 311 (59.0) 946 (85.1) 521 (64.2) 1.778 (72.6)
(a) Other: FF. AA, another system, none, does not known. (b) N˚ Home inhabitant. (c) Comorbility: the presence of the following pathologies: cancer, diabetes,
asthma, cardiopathology, EPOC, arterial hypertension, smoking, obesity, HIV/AIDS. (d) 2009 seasonal anti influenza vaccination history.
Table 3. Prevalence of the antibodies against the influenza A H1N1−2009 virus in the commnune San Felipe, according age stratum.
Blood Samples Odds Ratio (IC 95%)
Age Analized Positive
Prevalence % (IC 95%) (b)
Gross Adjusted (c)
0 - 4 393 127 29.8 (22.3 - 38.6) 6.59 (3.78 - 11.47) 6.13 (3.12 - 12.06)
5 - 18 1.028 306 30.2 (25.0 - 35.9) 6.71 (4.20 - 10.73) 6.70 (3.79 - 11.83)
19+ 768(a) 47 6.1 (4.2 - 8.7) 1.00 1.00
Total 2.189 (a) 480 13.5 (11.3 - 15.9)
(a) A subject gave an insufficient blood sample to give a result. (b) Prevalence calculated on the expanded sample. (c) Adjustment through the model of the
logistic regression (ENTER modality) that incorporated the age, gender, number of home inhabitants, health welfare, 2009 anti influenza vaccination history
and comorbility variables.

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 480
women than men, among homes of more than 5 people,
among affiliated to FONASA4 (as compared to those
affiliated to ISAPRE5 and among non vaccinated people
against seasonal influence, these differences did not
reach statistical signification (Table 4).
The prevalence among subjects that had influenza-
like6 symptoms between May-September 2009 was
32.2%; nonetheless, among people with positive anti-
bodies, the influenza-like history nearly reached 51.5%,
while it reached 30.3% among the people without anti-
bodies (Table 5).
There was no significant association between history
of vaccination against seasonal influenza and the pres-
ence of influenza-like symptoms; 33.6% of the people
with vaccination history presented influenza-like symp-
toms versus the 31.5% of people without vaccination his-
tory. Although, in all ages the proportion of people with
history of vaccination was bigger among those who had
influence-like symptoms as compared to those who did
not show any symptoms, the differences did not reach
Table 4. Odds ratio gross and adjusted of anti influenza A
H1N1-2009 positive serology for different variables (expanded
sample).
Odds Ratio (IC95%)
Va riable Positive
Serology % Gross Adjusted
a
Gender
Female 13.9 1.09 (0.74 - 1.61) 1.29 (0.86 - 1.92)
Male 12.9 1.00 1
N˚ of home
inhabitants
5 or more 19.7 1.98 (1.31 - 2.99) 1.25 (0.81 - 1.95)
4 or less 11.1 1 1
Welfare
Fonasa 14.6 1.50 (0.67 - 3.37) 1.19 (0.59 - 2.41)
Isapre 10.2 1 1
Vaccination 2009
Neg. 14.5 1.45 (0.90 - 2.32) 1.19 (0.70 - 2.03)
Pos. 10.5 1 1
Comorbilityb
One 9.8 0.54 (0.31 - 0.93) 1.25 (0.70 - 2.23)
Two or more 6.8 0.37 (0.16 - 0.82) 0.98 (0.38 - 2.51)
None 16.7 1 1
(a) Adjustment variables: age, gender, people in home, welfare, 2009 sea-
sonal anti influenza vaccination and morbility history. (b) Comorbility: the
presence of the following pathologies: cancer, diabetes, asthma, cardiopa-
thology, EPOC, arterial hypertension, smoking, obesity, HIV/AIDS.
statistical significance.
47% of subjects who reported influenza-like symp-
toms consulted for that cause. The proportion was higher
among those with influenza positive serology, reaching
73% (Table 6).
Among these, children under 5 years old were those
who showed the highest consultation rate (95%). How-
ever, the 19+ stratum showed a difference in the consul0
tation rate between the total sample and the seropositive
subjects.
Regarding the use of antiviral, 2.7% (95% CI; 1.9 - 3.9)
of subjects who answered the survey received antiviral
treatment, increasing to 11.5% (95% CI; 6.9 - 18.4)
among the seropositive subjects. In the first group (total
subjects surveyed), the percentage decreased with age
whereas among seropositive increased with age, reaching
15% among the 19+ year-old stratum (Table 6).
Whereas 8 sample subjects were hospitalized, only
one of them had anti-Influenza A(H1N1)pdm09 virus ti-
ters over 40.
4. DISCUSSION AND CONCLUSIONS
This is a population-based study representative of a Chi-
lean commune (San Felipe).
The attack rate of the first pandemic wave was 13.5%
in the same commune, with the highest rates among those
under 18 years old, confirming the relevance of this group
in the transmission of influenza in the community. Where-
as the prevalence of antibodies was higher among non
vaccinated people against seasonal influenza, it is not
possible to confirm the relationship between those vari-
ables. Seropositive people had more influenza-like symp-
toms, consulted more and received more antiviral than
the average population.
The results of this study showed that 73% of seroposi-
tive people consulted due to influenza-like symptoms
and only 11% of them say they received antiviral treat-
ment. This fact is outstanding considering the protocol
instructed by the Ministry of Health who stated that all
the people who consulted because of influenza-like symp-
toms would receive free antiviral treatment.
Considering in San Felipe age ETI rates and antiviral
delivery were very similar to the national average, it is
possible to assume that those results can be generalized
to the rest of the country (Table 7).
According to this study, the pyramid of disease sever-
ity in San Felipe had a base of 13.5% people infected
serologically (representing 10.181 people), 51.5% of the
people with positive serology had symptoms caused by
pandemic A(H1N1) influenza (which represents 7% of
the total population); 73% of them consulted because of
the symptoms (representing 5.1% of the commune popu-
lation). Finally, 8 people from the sample were hospital-
ized for ILI, and only 1 had positive serology; there were
4Public health system.
5Private health system.
6Those subjects who answered positive the question: Did you have
fever with coryza or cough between May 1 and September 2009, were
considered as symptomatic.

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 481
Table 5. Influenza like symptoms among seropositive and seronegative subjects.
Age With Symptoms %
(IC; 95%) (a) Seropositives (%) (a) Seronegatives (%) (a)
With Symptoms Without SymptomsWith Symptoms Without Symptoms
0 - 4 58.2 (50.3 - 65.7) 59.4 40.6 56.5 43.5
5 - 18 46.8 (41.1 - 52.5) 55.4 44.6 43.3 56.7
19+ 24.3 (19.8 - 29.6) 40.7 59.3 24.8 75.2
Total 32.2 (28.5 - 36 - 1) 51.5 48.5 30.3 69.7
(a) Prevalence estimated over the expanded sample.
Table 6. % of consultations because of IT and use of antivirals among the total population surveyed and population with positive
serology.
% consultations because of IT (a) % antiviral use (a)
Age (years)
Subjects Surveyed Seropositive Subjects Subjects Surveyed Seropositive Subjects
0 - 4 88,8 (80.0 - 94.1) 95.8 (89.6 - 98.4) 4.7 (2.0-10.3) 5.8 (2.8 - 11.9)
5 - 18 57.2 (50.7 - 63.5) 66.1 (53.2 - 76.9) 5.2 (3.2 - 8.4) 11 (5.4 - 21.2)
19+ 36.6 (29.1 - 44.8) 73.9 (56.2 - 86.2) 1.7 (0.9 - 3.1) 15 (6.3 - 31.5)
Total 46.9 (41.5 - 52.4) 73.1 (63.9 - 80.6) 2.7 (1.9 - 3.9) 11.5 (6.9 - 18.4)
(a) Prevalence estimated over the expanded sample.
Table 7. Estimation at national level on the theoretical number of subjects with anti influenza A H1N1−2009 serology, according to
age stratum.
Populationa Seroprevalenceb Theoretical Cases
Age
n % n
29.8 372.001
0 - 4 1.248.325
(22.3 - 38.6) (278.377 - 481.854)
30.2 1.224.534
5 - 18 4.054.748
(25.0 - 35.9) (1.013.687 - 1.455.655)
6.1 719.263
19+ 11.791.197
(4.2 - 8.7) (495.231 - 1.025.835)
2.315.798
Total 17.094.270 -
(1.787.295 - 2.963.344)
(a) Estimated Population (Chile 2010). (b) Prevalence calculated in the present study for the commune of San Felipe, Value between brackets: IC 95%.
no deceased people in the sample.
World studies on antibody prevalence against 2009 A
(H1N1) influenza show variable results. So, it is possible
to find a 6% in Pune, India [5], 15.7% among pregnant
women in Manitoba, Canada [6], 18.2% among under 15
years old in England [7], 18.8% among non vaccinated
people against H1N1 in China [8], 29.5% in New Zea-
land [9], 28.4% in Australia [10], 29.7% in Taiwan [11].
The study carried out in the San Felipe commune shows
an antibody prevalence relatively low as compared to
other studies that have measured post pandemic preva-
lence of antibodies against influenza A(H1N1)pdm09 vi-
rus that could be partly explained by differences in the
methodology to obtain the samples. In San Felipe it had a
representative sample of the population base.
On the other hand, studies suggest that there would be
a proportion of the population, especially older adults,
with pre-existing immunity to the pandemic H1N1 virus
(England), due to prior exposure to antigenically related
viruses related to the influenza H1N1 virus that circu-

O. Andrea et al. / World Journal of Cardiovascular Diseases 3 (2013) 476-482 482
lated the first half of the twentieth century. This pre-ex-
isting immunity varies from 0.9% in Pune, India [5],
1.8% among under 5 years old to 31% among 80 year-
old adults (England) [7]; 12.8% in New South Wales,
Australia [10], 7.1% in Canada [11], 11.9% in New Zea-
land [9], 45% among >80 year-old subjects [12], 24%
among >65 year-old subjects [13].
The latter estimate was not obtained in San Felipe, so
obtained results could overestimate the real impact that
the first pandemic wave had in that commune. On the
other hand, it gives important information in terms of the
structure of the pandemic severity, from subclinical in-
fection and asymptomatic until the SARI. Furthermore, it
gives evidence about the necessary considerations when
interpreting the results of the epidemiological surveil-
lance of influenza.
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