“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
Video 5.5: The Host: Age
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From the lesson
Theme Two: Spread (Epidemiological triangle)
Theme One descriptions
Taught By
Gabriel M. Leung (HKU)
Dean
Joseph T. Wu (HKU)
Associate Professor
Kwok-Yung Yuen (HKU)
Professor
Tommy Lam (HKU)
Assistant Professor
Maria Huachen Zhu (HKU)
Associate Professor
Guan Yi (HKU)
Chair Professor
Benjamin Cowling (HKU)
Professor
Thomas Abraham (HKU)
Associate Professor
Mark Jit (LSHTM)
Professor
Malik Peiris (HKU)
Chair Professor
Marc Lipsitch (Harvard)
Professor
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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