Epidemiology of seasonal

influenza in Hong Kong and use of

seasonal influenza vaccines

Dr Albert Au

30 Sep 2019

Overview of seasonal influenza in HK

• Usually 2 influenza seasons each year

– Main season: winter season occurring between

Jan and Mar/Apr

• Lasted for 12 - 17 weeks in past 5 years (except

7 weeks in the atypical 2016/17 winter season)

– Another summer season with variable timing

(between Jun and Sep)

• Lasted for 5 - 8 weeks in past 5 years (except 16

weeks in the atypical 2017 summer season)

• not occurred in some years (e.g. no summer

season in 2018 & 2019)

2018/19 winter influenza season in HK

• Started in early Jan this year

• Overall seasonal influenza activity had been increasing

rapidly to a very high level during mid to late Jan

• Started to decrease since Feb and returned to the baseline

level in early Apr

• Lasted for 14 weeks (12-17 weeks in major flu seasons in

past few years)

• Young children were particularly affected in this season

– Large number of influenza-like illness (ILI) outbreaks in

kindergartens/ child care centres (KGs/CCCs)

– Hospitalisation rate in public hospitals was highest in

young children aged < 6 years

Laboratory surveillance

• Among the respiratory specimens received, the positive

percentage of seasonal influenza viruses peaked at 30.1%

in mid-Jan (peak recorded during 2015–2018 ranged from

26.4% to 40.9%)

• The predominant virus was influenza A(H1) (accounting for

75% of all influenza detections)

ILI outbreaks in schools/institutions• ILI outbreaks markedly increased to very high intensity level during

mid to late Jan (121 - 209 outbreaks per week)

• 863 ILI outbreaks were recorded in this season, which was the

highest number recorded since the 2009 pandemic

Type of

schools/institutions

No. of outbreaks

(% among all

outbreaks)

Total no. of

schools/

institutions in

HK

Percentage of

schools/

institutions

affected

KGs/CCCs 528 (61.2%) 1,063 50%

Primary schools 183 (21.2%) 587 31%

Secondary schools 35 (4.0%) 524 7%

RCHEs 55 (6.4%) 747 7%

RCHDs 19 (2.2%) 319 6%

Others 43 (5.0%) -- --

Total 863 -- --

Cumulative numbers of reported

ILI outbreaks2018/19 winter: Start of early break for Lunar New

Year for KGs/CCCs on 26 Jan (at the end of the

4th week)

Influenza-associated admission

rates in public hospitals• Peaked in the week ending Jan 19 (1.59 per 10,000 population)

[within the range of 0.67 to 1.91 recorded in major seasons during

2015-2018]

• Peak weekly admission rates in this season by age groups:

highest among young children 0-5 years (11.07 per 10,000

population), followed by elderly ≥65 years (3.10) & children 6-11

years (1.99)

Children < 6 years

Adult influenza cases who required

ICU admission or died• Total 601 cases (including 356 deaths) in this season

• Most (64%) affected elderly ≥65 years；about 78% had pre-existing

chronic diseases

• Only 157 (26%) were known to have received 2018/19 seasonal

influenza vaccine (SIV)

• About 26% were aged 50-64 years, higher than H3N2-predominant

seasons (e.g. 15% in 2017 summer) and B-predominant seasons (e.g.

20% in 2017/18 winter)

Age

group

All cases including deaths Deaths among the cases

No. of cases

(%)

Cumulative

incidence (per

million population)

No. of

deaths

(%)

Cumulative mortality

(per million

population)

18 - 49 62 (10.3%) 18.4 6 (1.7%) 1.8

50 - 64 154 (25.6%) 86.0 42 (11.8%) 23.4

≥65 385 (64.1%) 304.1 308 (86.5%) 243.2

Total 601 93.6 356 55.4

*Not including 2 children under 6 months old

Paediatric influenza-associated

severe complications/deaths

• 24 cases (including 1 death) were recorded in this season

• Age range: 1 month – 16 years (median: 4 years)

• 6 (25%) had pre-existing chronic diseases

• 16 (73%*) had not ever received the 2018/19 SIV

Age groupNo. of cases

(death among the cases)

Cumulative incidence

(per million population)

0 – 5 13 (1) 38.8

6 - 11 8 (0) 21.8

12 - 17 3 (0) 9.2

Epidemiology of Seasonal Influenza

in Hong Kong

Distribution of ILI outbreaks

by types of institutions

Most outbreaks occurred in RCHEs

during 2014/15 winter & 2017 summer

seasons predominated by A(H3N2)

Most outbreaks occurred in schools during

2015/16, 2017/18 & 2018/19 winter

seasons predominated by A(H1N1) or B

H3N2 H1N1 & B H3N2 B H1N1

Weekly influenza-associated

admission rates in HA hospitals

• Highest in young children <6 years in most seasons, followed by elderly

≥65 years, and then children 6-11 years

• The rate among elderly was higher during H3N2 seasons

Season

(predominating

virus)

Peak admission rate (per 10,000 population) recorded

0-5 6-11 12-17 18-49 50-64 ≥65 All ages

2018/19 winter

(H1)11.07 1.99 1.13 0.56 1.06 3.10 1.59

2017/18 winter

(B)8.81 3.62 1.50 0.36 0.87 4.06 1.50

2017 summer

(H3)9.09 1.65 0.61 0.31 0.87 6.36 1.91

2015/16 winter

(H1&B)6.15 1.79 0.38 0.17 0.38 1.04 0.67

2014/15 winter

(H3)2.78 1.26 0.42 0.16 0.39 5.34 1.17

Cumulative incidence rates of severe cases

(per million population)

• Much higher in elderly than other age groups

• Second high was 50 - 64 years, followed by young

children 0 - 5 years

503.3

187.4

379.9

326.2 304.1

Surveillance of adult influenza cases

who required ICU admission or died

• 409 – 647 adult severe cases per season (211- 501 deaths)

• About 80% had pre-existing chronic medical diseases

• Only about 30% were known to have received SIV for the respective

seasons

Season

(predominating virus)

No. of

weeks

No. of severe

cases including

deaths

No. of

deaths

2018/19 winter (H1) 14 601 356

2017/18 winter (B) 12 570 382

2017 summer (H3) 16 582 430

2015/16 winter (H1&B) 17 409 211

2014/15 winter (H3) 17 647 501

Age distribution of adult severe cases(2015 – 2019)

All severe cases (ICU admissions or deaths) (N = 2724)

Deaths: (N = 1811)

H3N2 B H1N1

18-49 1.1% 1.5% 2.7%

50-64 5.9% 11.6% 19.4%

≥65 93.0% 86.8% 77.9%

Remark: Excluding cases infected with A(untyped), C, and co-infection

H3N2 B H1N1

18-49 3.7% 6.0% 14.9%

50-64 11.0% 18.7% 35.5%

≥65 85.3% 75.3% 49.6%

Surveillance of severe paediatric

influenza cases (2015 – 2019)

• 18 – 27 cases per season (1 – 3 deaths)

• About 31% had pre-existing chronic medical diseases

• Only 14% were known to have received SIV for the respective

seasons

Season

(predominating virus)

No. of

weeks

No. of cases

including deaths

No. of

deaths

2018/19 winter (H1) 14 24 1

2017/18 winter (B) 12 20 2

2017 summer (H3) 16 19 3

2015/16 winter (H1/B) 17 27 3

2014/15 winter (H3) 17 18 1

Complications of severe paediatric

influenza cases (2015 – 2019)

ComplicationsNo. of cases

including deaths (%)

No. of deaths

(%)

Neurological

complications

(encephalitis,

encephalopathy, etc.)

65 (46.8%) 11 (78.6%)

Severe pneumonia 54 (38.8%) 1 (7.1%)

Shock 20 (14.4%) 1 (7.1%)

Sepsis 7 (5.0%) 1 (7.1%)

Myocarditis 6 (4.3%) 1 (7.1%)

Total 139 (100%) 14 (100%)

Note: Some cases had > 1 complications.

Summary

• Significant disease burden of seasonal influenza during

influenza seasons in terms of institutional outbreaks,

hospitalisations and mortality, esp. among elderly and

young children

• H3N2 tends to affect elderly while H1N1 tend to affect

children more

• The majority of severe cases did not receive SIV

• Promoting SIV is the most effective means for mitigating

the disease burden of seasonal influenza

Seasonal Influenza Vaccine

(SIV)

● an A/Brisbane/02/2018 (H1N1)pdm09-like virus

● an A/Kansas/14/2017 (H3N2)-like virus

● a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage)

● a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage)

The influenza B virus component of trivalent vaccines should be a

B/Colorado/06/2017-like virus of the B/Victoria/2/87-lineage

Recommendation on seasonal

influenza vaccine composition in

2019/20 (Northern hemisphere)

20

Compositions of the SIVs

recommended by WHO

21

Seasonal influenza vaccines

available in HK in 2019/20 season(Information from Drug Office, DH)

22

Quadrivalent inactivated influenza vaccines (IIV)

• Vaxigriptetra Vaccine 0.5ml (6 mths or above)

• Fluarix Tetra Northern Hemisphere Vaccine Suspension

for Injectioni(6 mths or above)

• Influvac Tetra Vaccine Suspension for Injection (Northern

Hemisphere)

Live attenuated influenza vaccine (LAIV)

• Flumist Quadrivalent Influenza Intranasal Vaccine (2-49

years)

Recommendations on influenza

vaccination in 2019/20 season● All members of the public aged 6 months or above except those with

known contraindications should receive SIV annually for personal

protection

● People who are in the priority groups are generally at increased risk of

severe influenza or transmitting influenza to those at high risk. Therefore,

they shall have higher priority for SIV

1. Pregnant women

2. Elderly persons living in residential care homes

3. Long-stay residents of institutions for persons with disability

4. Persons aged 50 years or above

5. Persons with chronic medical problems

6. Health care workers

7. Children aged 6 months to 11 years

8. Poultry workers

9. Pig farmers and pig-slaughtering industry personnel

23

Types of SIVs recommended

to be used in HK

• Both IIV and LAIV are recommended

• For IIVs, quadrivalent IIV is preferred to trivalent IIV

due to the additional protection against one more

lineage of influenza B offered by quadrivalent IIV.

Depending on individual brand, IIVs are

recommended for use among people aged six

months of age or older

• For LAIV which is a quadrivalent SIV, it can be used

for people 2-49 years of age except those who are

pregnant, immunocompromised or with other

contraindications

Contraindications

• All SIV: history of severe hypersensitivity to any of the vaccine

components or a previous dose of SIV

• Additional contraindications for LAIV:

– Concomitant aspirin or salicylate-containing therapy in children

and adolescents;

– Children 2 years through 4 years who have asthma or who have

had a history of wheezing in the past 12 month;

– Children and adults who are immunocompromised due to any

cause;

– Close contacts and caregivers of severely immunosuppressed

persons who require a protected environment;

– Pregnancy; and

– Receipt of influenza antiviral medication within previous 48 hours

About egg allergy (both IIV & LAIV)

• SIV contains ovalbumin (a chicken protein), but the

manufacturing process involves repeated purification

and the ovalbumin content is very little

• Even people who are allergic to eggs are generally safe

to receive vaccination

– Individuals with mild egg allergy can receive SIV in

primary care setting

– Individuals with a history of anaphylaxis to egg should

have SIV administered by health care professionals in

appropriate medical facilities with capacity to

recognise and manage severe allergic reactions

• A single dose of SIV is the standard regimen for persons ≥9 years

• Children below 9 years:

• vaccine-naïve: 2 doses of SIV with an interval of at least 4 weeks

• who have received one or more doses of SIV before: 1 dose

• Inactivated and live vaccines may be administered simultaneously or at any interval between doses

• For individuals receiving LAIV, other live vaccines not administered on the same day should be administered at least 4 weeks apart

Dosing schedule and

vaccination interval

27

Estimates of Vaccine Effectiveness of

SIV at Primary Care Setting in HK

(2018/19 Winter Season)

No. of specimens collected from

private medical practitioners

Respiratory specimens collected

from ILI patients

(n=1,419)

Study period

Dec 2018 to Jun 2019

Specimens excluded: 76

• Unknown SIV history and/ or

chronic disease status: 72

• < 6 months of age: 3

• Influenza C: 1Respiratory specimens

analysed

(n=1,343)

Influenza-positive cases

•Any influenza A/B (n=690)

•Influenza A(H1) (n=389)

Influenza-negative

controls

(n=653)

Estimates of vaccine effectiveness

of SIV

^Adjusted for age, time of specimen received (week) and chronic

medical illness

Adjusted vaccine effectiveness^

All influenzaInfluenza A

(H1)

Influenza A

(H3)Influenza B

51.3% 60.9% 42.9% 51.4%

In the 2018/19 season, the effectiveness of SIV in prevention of

ILI caused by laboratory-confirmed influenza locally was about

50-60%, which was similar to the estimates from other areas

(45-72%)

Comparison with estimates for 2018/19

season in overseas countries

Country/Region VE against A(H1) (95% CI)

Canada 72% (60 to 81)

Europe 71% (38 to 86)

United Kingdom 57% (20 to 77)

Denmark 55% (41 to 65)

United States 44% (36 to 51)

Spain 45% (-20 to 75)

• Interim early season estimates in primary care setting

in overseas countries ranged from 45% to 72% against

A(H1N1) predominated in 2018/19 Northern

Hemisphere winter season

• SIV could offer moderate to good protection in this

season

School typeWithout outreach

vaccination

With outreach

vaccination

KGs/CCCs

(n=1,063)

394 / 879

(44.8%)

51 / 184*

(27.7%)

Primary schools

(n=587)

66 / 184

(35.9%)

76 / 403*

(18.9%)

Proportion of schools with ILI

outbreaks in 2018/19 winter influenza

season

38%

47%

* Note: Schools having outbreaks within 2 weeks after outreach vaccination are not

counted (1 KG/CCC & 1 primary school). One primary school with outreach SIV for

staff only was also not counted.

Thank you