ICU Management & Practice, ICU Volume 12 - Issue 4 - Winter 2012/2013
Managing Population Immunity for Vaccine Preventable Diseases
One lesson from progress towards polio
eradication suggests that using models and measurements together to manage
population immunity may play a key role in supporting the paradigm shift
required to value prevention and realise the full benefits of vaccines
(Thompson et al. 2012b). This paper discusses individual and population
immunity, prevention as a choice, valuing prevention, and some opportunities
for intensive care unit (ICU) managers to further contribute to improving
global health.
Introduction
Thanks to the miracle of vaccines, healthcare providers on the
front lines continue to see progressively fewer cases of vaccine-preventable
diseases (VPDs). Today, most medical students in developed countries graduate
and expect to practice without seeing a single case of paralysis caused by
polio, and their experience with many once-common VPDs remains limited to what
they learnt from curriculum materials.
While most individuals in most developed countries enjoy significant protection from VPDs, the same is not true in all developing countries. Recognising both the inequity and opportunity presented by the current situation, the Global Vaccine Action Plan (GVAP) aspires to extend the full benefits of immunisation to all people by 2020, to create a world “in which all individuals and communities enjoy lives free from vaccine-preventable diseases” (WHO, 2012). The question remains, however, “how will we get from here to there?”
Individual and Population Immunity
Vaccines, although not entirely risk-free, provide a safe and
effective way to prevent cases of disease before they occur by enabling the
immune systems of individuals to avoid or better fight infections upon
exposure. Immunisation protects individuals, at least until the immunity wanes
or the infectious agent changes. Population immunity reflects the integration
of individual immunity over all people in the community. In the case of polio,
we define population immunity as the overall level of protection from poliovirus transmission within a population, which
focuses on the prevention of infections and goes beyond simply focusing on the
cases of disease (Thompson et al. 2012b). High levels of population immunity
effectively provide a barrier that inhibits sustained viral transmission, such
that transmission stops once population immunity exceeds the threshold required
for any introductions of the disease to die out. Levels of population immunity
above the threshold protect unvaccinated individuals (e.g. those younger than
the recommended age of the first dose, those with contraindications or vaccine
failures, and those who miss vaccination unintentionally or intentionally), at
least so long as the population maintains sufficiently high levels. The actual
thresholds vary by population and conditions. Consequently, some populations
need to achieve and maintain relatively higher levels of population immunity than
others. This means that interventions that suffice in some areas fall short in
others. For example, routine immunisation alone may work for some countries, while
other countries may need to conduct supplemental immunisation activities.
The
concept of population immunity may seem simple and obvious, but its
characterisation can prove very challenging. We cannot directly observe or
measure population immunity. Health systems and immunisation programmes
typically monitor levels of routine immunisation coverage as a proxy for
population immunity, but this metric only reveals one important part of the
overall story. Similarly, while detecting cases as part of disease surveillance
will indicate the lack of sufficient population immunity, the absence of cases
does not provide information about potential risks or the accumulation of
susceptible individuals. Serological studies provide important information
about the individuals included in samples, but this only offers a snapshot
view.
The
ability of a population to sustain transmission depends on the integration of
the entire population, and we must understand population immunity as a dynamic
“stock” (i.e. a level of overall protection from infection that changes with
time). The level of population immunity increases as individuals get vaccinated
or recover from infection with the VPD, and the level decreases as non-immune
individuals enter the population, immune individuals die, or immunity wanes. In
this context, individuals who miss vaccination matter (e.g. migrants, certain
age cohorts skipped due to a disruption in supply, etc.), because susceptible
individuals can accumulate and participate in transmission if and when the
disease occurs.
Development
of a population immunity model along with the collection of measurements of
current and historical vaccine coverage, demographics, and other factors makes
it possible to characterise and visualise population immunity (or
vulnerability). For polio, modelling population immunity facilitates
consideration of the immunological implications of prior exposure to any
circulating live polioviruses and/or vaccination with a live oral poliovirus
vaccine (OPV) or injected inactivated poliovirus vaccine (IPV) (Duintjer
Tebbens et al. 2005; Thompson et al. 2012b). Successful immunisation with
either vaccine protects individuals from disease (i.e. paralytic polio), but
even immune individuals can potentially become reinfected and participate
asymptomatically in poliovirus transmission to some degree, with their
participation likely increasing with time due to waning. The use of a model
also captures the differences in how IPV and OPV work. For example, as a live
virus, OPV infects vaccine recipients, which stimulates mucosal immunity and
leads to the excretion of live polioviruses that can then cause secondary
infections. In contrast, the relatively more expensive IPV does not cause
secondary circulation or infection (i.e. it protects only the vaccine
recipient). IPV also does not cause the relatively rare but real cases of
vaccine-associated paralytic polio, which has made it a costly but attractive
alternative to OPV for developed countries. Models can help us characterise the
risks and consider the impacts of potential risk management options before we
make a decision and take action. Similarly they can help to demonstrate the
consequences of inaction, which is also a choice.
Choosing Prevention
Choosing to eradicate a disease represents the ultimate in
prevention. Eradication presents a unique opportunity to protect current and
future generations, and it requires stopping chains of transmission everywhere
and maintaining this state. We can only eradicate the diseases that we can
meaningfully stop from being transmitted and for which we can coordinate and
cooperate globally. To date, global health systems successfully stopped the
human transmission of smallpox, wild poliovirus serotype 2, and the
SARS virus that circulated between November 2002 and July 2003 (Thompson and
Duintjer Tebbens 2011). We also recently celebrated the global eradication of
rinderpest, an animal virus similar to measles that led to devastating impacts
on herd animals and food supplies. The global eradication of wild poliovirus
serotypes 1 and 3, though as yet elusive, now appears within reach. The
possibility of eradicating measles and rubella continues to emerge as a topic
of discussion, particularly with successful elimination in the Americas and
measles elimination goals in place in four of the five remaining WHO regions
(WHO 2011; WHO 2012b).
The ability to use a model to characterise and manage population immunity represents a game changer for disease control and prevention. Since we can use models to make our choices and their impacts transparent, they can help us anticipate the consequences of our actions and manage expectations. As we approach the final stages of polio eradication, managing population immunity is emerging as the key to success. The case-based strategy of testing samples from patients that present with acute flaccid paralysis (AFP), which identifies symptomatic poliovirus infections after they have caused paralysis, does not provide an opportunity to identify immunity gaps before outbreaks occur. Eradication means preventing all future cases before they occur. Since polioviruses can circulate asymptomatically, eradication requires the use of a tool that supports the objective of no anticipated cases, while we also still actively use AFP surveillance to ensure that no paralytic polio cases actually occur from exposure to wild polioviruses. The models help us characterise the benefits of prevention, because they allow us to count the cases that do not occur and to give credit to prevention activities.
Valuing Prevention
Any healthcare provider who has treated a complicated case of
measles, seen a child born with congenital
rubella syndrome, provided respiration for a patient paralysed by polio,
watched a child with pertussis whoop, or managed a serious case of any other
VPD can easily appreciate the benefits of preventing these bad outcomes before
they occur. Vaccines provide significant health and financial savings, and they
represent some of the most cost-effective medical
interventions available. However, immunisation requires the investment of
resources, and they pose some risks. Sustained investments in population
immunity depend on the perception of need, and successful immunisation
programmes have dropped the burdens of disease to such low levels that even
some healthcare providers may not recognise the critical role that vaccines
continue to play in achieving and sustaining community and global health. Those
without direct experience of outbreaks may find it difficult to understand the
dynamics of infectious diseases and the reality that they can find susceptible
individuals and spread devastatingly fast.
In the
western hemisphere, progress in infectious disease control and prevention in
combination with national and regional commitments to the elimination of polio,
measles, and rubella have created a new normal. Most individuals and
populations in the Americas expect complete prevention of VPDs. Health
ministers of countries in the Americas sustain their investments in
immunisation and hold each other accountable for importations. When informed by
front line healthcare providers about even a single case of polio, measles, or rubella,
health authorities in the Americas view the case as an indication of system
failure and as a signal that emergency action is required. However, the shift
to prevention represents a significant change around the world. For example, recent
outbreaks of measles in Europe suggest that the disease is still tolerated as a
normal and acceptable sickness, even though measles has caused preventable and
tragic deaths. Significant drops in vaccine coverage in Ukraine, due to vaccine
scares and politicisation of immunisation, may actually present a threat to the
entire European region and its commitment to eliminate measles and rubella. If
we want to achieve the full potential benefits of vaccines globally, then this
will require a permanent paradigm shift to one that values prevention of VPDs.
Shifting towards prevention does not necessarily mean eradication for all VPDs,
but it means not tolerating cases or bad management of these diseases. For example,
while we cannot remove all Clostridium
tetani from the environment, with good management we
can eliminate all cases of tetanus, including neonatal tetanus.
Perhaps
the largest challenge to achieving and maintaining global health relates to the
lack in Ukraine, due to vaccine scares and politicisation of immunisation, may
actually present of sustained commitments and investments for prevention. We
all know that individuals often do not receive credit for their contributions
to endevours that prevent bad things from happening, because it is hard to
count the cases that do not occur. In addition, the limited resources allocated
to prevention activities may get shifted to managing the crisis of today
instead of preventing the crises of tomorrow. This unfortunately creates a
vicious cycle, because prevention often represents a more cost-effective
option.
Opportunities for ICU Managers
All healthcare providers and health systems play important roles.
At the individual level, they can monitor the immunisation status of individual
patients and ensure that patients receive vaccinations on schedule or catch up on
any they miss. The actions of individual providers collectively impact the
health of the population, and it matters if providers pursue a goal of fully protecting
100% of their patients from VPDs.
Front line healthcare providers play a critical role in
communicating the benefits of vaccines in their communities. Practicing
providers should find ways to train medical students, residents and others to
recognise and manage cases of VPDs. We must all appreciate the disruptive impact
of VPD cases on health systems and the need for preparedness for managing
outbreaks. The phrase, “preparing for the worst while managing for the best” is
a powerful reminder of why prevention makes a difference. Particularly in the
absence of cases, front line providers must play a critical role in advocating vaccination and helping patients and their communities to recognise
the benefits of immunisation and prevention. Patients need to understand that
it is not too late to get vaccinated against many VPDs and to know that if they
do not get vaccinated then they remain at risk for contracting a serious
disease.
Front line providers may also play an important would send an important message that the provider community values immunisation and prevention. Those who treat the cases that result when the system fails contribute powerfully to individual immunity and prevention in their immediate surroundings and all of the individual actions aggregate to global health.