Video 5.5: The Host: Age

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From the course by The University of Hong Kong

Epidemics

25 ratings

The University of Hong Kong

Epidemics

25 ratings

“If history is our guide, we can assume that the battle between the intellect and will of the human species and the extraordinary adaptability of microbes will be never-ending.” (1) Despite all the remarkable technological breakthroughs that we have made over the past few decades, the threat from infectious diseases has significantly accelerated. In this course, we will learn why this is the case by looking at the fundamental scientific principles underlying epidemics and the public health actions behind their prevention and control in the 21st century. This course covers the following four topics: 1. Origins of novel pathogens; 2. Analysis of the spread of infectious diseases; 3. Medical and public health countermeasures to prevent and control epidemics; 4. Panel discussions involving leading public health experts with deep frontline experiences to share their views on risk communication, crisis management, ethics and public trust in the context of infectious disease control. In addition to the original introductory sessions on epidemics, we revamped the course by adding: - new panel discussions with world-leading experts; and - supplementary modules on next generation informatics for combating epidemics. -------------------------------------------------------- (1) Fauci AS, Touchette NA, Folkers GK. Emerging Infectious Diseases: a 10-Year Perspective from the National Institute of Allergy and Infectious Diseases. Emerg Infect Dis 2005 Apr; 11(4):519-25. What you'll learn - Demonstrate knowledge of the origins, spread and control of infectious disease epidemics - Demonstrate understanding of the importance of effective communication about epidemics - Demonstrate understanding of key contemporary issues relating to epidemics from a global perspective

From the lesson

Theme Two: Spread (Epidemiological triangle)

Theme One descriptions

Week 5 Introduction0:51

Video 5.1: Epidemiologic Triangle1:08

Video 5.2: The Pathogen: The effects of dose and pathogen genetics5:13

Video 5.3: The Pathogen: Evolution9:29

Video 5.4: The Host: Overview5:00

Video 5.5: The Host: Age7:33

Video 5.6: The Host: Sexual mixing4:45

Video 5.7: The Environment7:54

Meet the Instructors

Gabriel M. Leung (HKU)
Dean
Li Ka Shing Faculty of Medicine
Joseph T. Wu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Kwok-Yung Yuen (HKU)
Professor
Li Ka Shing Faculty of Medicine
Tommy Lam (HKU)
Assistant Professor
Li Ka Shing Faculty of Medicine
Maria Huachen Zhu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Guan Yi (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Benjamin Cowling (HKU)
Professor
Li Ka Shing Faculty of Medicine
Thomas Abraham (HKU)
Associate Professor
Journalism and Media Studies Centre
Mark Jit (LSHTM)
Professor
Department of Infectious Disease Epidemiology
Malik Peiris (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Marc Lipsitch (Harvard)
Professor
Department of Epidemiology

In this session, we will focus on age

and its effects on disease transmission and severity.

In the previous session, we mentioned that age

is the most important attribute of a host

from the perspective of infectious diseases.

The figure on the slide shows one reason for this:

infection with a particular disease can be

more or less severe depending on the age

at which a person is infected.

For many infections, the case-fatality ratio,

or risk of dying if infected, is greatest for people

infected at young age, as shown in this figure.

Note that the axes are logarithmic, so each

gradation is 10 times more fatal than the one below.

For these diseases, protecting infants from infection

is a particularly high priority, and fortunately

we have excellent vaccines for each of them,

though the resurgence of pertussis in some parts

of the world is an area for concern.

Some other infections are particularly dangerous

when contracted at older ages.

Chickenpox, or varicella virus infection, is only

rarely fatal but it is more likely to fatal in adults and

older adolescents as well as in infants.

In the case of rubella, infection is rarely

severe for the infected person,

no matter what their age.

but we are concerned about rubella because

if a pregnant mother is infected

it can cause severe health consequences

for the fetus she is carrying.

Therefore, a goal of public health policy is to prevent

rubella cases in women of childbearing age,

while prevention in males and in females of

other ages is a lower priority.

Host age also affects both the likelihood

of becoming infected and the likelihood

of passing on infection to others.

Mainly, this is because people of different ages

have different numbers and different

types of contacts each day, and these contacts

each have some probability of leading

to the transmission of disease.

These graphs show the numbers of contacts per day

between people of different ages in four

European countries, with blue being low

and white being high.

You can see that the largest numbers of contacts

in white are among persons under 20 or so

and that most of these contacts are with

people of about the same age.

Contacts between individuals of the same age

are shown on the diagonal.

You can also see what look like green wings

on the graph which show contacts between people

of parenting age and children.

Prior to this study, called the POLYMOD study,

epidemiologists had inferred from incidence rates

in different age groups, that contacts were more

frequent among children than adults and

that they were assortative, meaning that

individuals preferentially contact

those of a similar age.

POLYMOD confirmed this based on survey data

and gave numbers to the specific mixing patterns.

while also allowing comparisons across countries.

These contact rates affect both an individual’s

likelihood of getting infected and an individual‘s

ability to pass on infection once they become infected.

When one group in a population is both more likely

to get infected and more likely to pass infection on,

that group is sometimes called a core group

for transmission of the disease involved.

Core groups are even more likely to form,

if contacts are assortative, meaning that most

contacts that transmit disease transmit within

the core group rather than spilling out into other groups.

While epidemiologists differ in their precise

definition of a core group, the fundamental idea

is that it is the group of persons in the population

who sustain transmission while other groups

most often become infected through a chain of

transmission that can be traced

back to the core group.

For example, in infections transmitted mostly by

close contact or by respiratory contact,

children are often a core group, because they

contact one another frequently and transmit

the infection among themselves, but they also

contact adults in some cases infecting those adults.

This notion is useful in thinking about control

measures, as it may be possible to reduce the

burden of disease considerably in the population

as a whole just by vaccinating the core group.

Starting in 2000, the pneumococcal conjugate

vaccine, called PCV7, was given to infants

and toddlers in the United States.

This graph shows the decline in invasive disease from the

types of pneumococcus included in the vaccine

in the years following introduction by age group.

At left is the age group that actually received

the vaccine, which had the largest burden of disease

and also showed the biggest reduction

with vaccine introduction.

But disease declined in all other age groups too

because the youngest children in the population

were a core group for transmission

and the infection spilled out from that group.

Epidemiologists partitioned the averted cases -

cases that didn’t happen because of infant

and toddler vaccination - into those that could

be attributed to direct protection in the vaccine

recipients and those that were in unvaccinated

people who were protected indirectly by herd immunity.

About 2/3 of the averted cases were attributed

to herd immunity, even though only a small

fraction of the population

about five years worth of birth cohorts,

had been vaccinated.

When a particular age group is both very important

in transmitting an infection and also at high

risk of severe consequences when infected,

it clearly makes sense to vaccinate that group.

In the case of pneumococcal disease, infants

and toddlers are one of the two age groups,

along with the elderly, that get severe disease

and they are also key to transmission

so they were a natural choice for vaccination.

In other cases, the core age groups that maintain

transmission may not be those at

greatest risk of severe consequences.

For example, influenza infection in children over two

is rarely life-threatening but it poses a high risk

to the elderly adults or people with other

predisposing conditions, such as certain heart

and neurological conditions.

In this case, there are two different approaches to

protecting the persons at high risk.

The first is to vaccinate the members of

the high-risk group.

The second is to vaccinate a large fraction of

the core group in an effort to reduce transmission

and protect the high-risk group indirectly.

A randomized controlled trial of influenza vaccination

in Canada estimated that vaccinating children

against flu could reduce the incidence

of flu in adults by 39 percent,

even though the adults themselves

did not receive flu vaccine.

An additional complication is that elderly people,

the ones that would benefit most from immunity

to flu, don’t make a very strong immune response

to the vaccine, so vaccinating them may

not give them full protection.

For this reason, a combined strategy of using

influenza vaccine for high risk groups

and also for children who are key

to transmission is increasingly popular.

In practice, this means recommending

flu vaccination to everyone.

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