Control of poliomyelitis by pulse immunisation in Vellore, India

Epidemiological and evolutionary consequences of periodicity in treatment coverage

Host heterogeneity is a key driver of host-pathogen dynamics. In particular, the use of treatments against infectious diseases creates variation in quality among hosts, which can have both epidemiological and evolutionary consequences. We present a general theoretical model to highlight the consequences of different imperfect treatments on pathogen prevalence and evolution. These treatments differ in their action on host and pathogen traits. In contrast with previous studies, we assume that treatment coverage can vary in time, as in seasonal or pulsed treatment strategies. We show that periodic treatment strategies can limit both disease spread and virulence evolution, depending on the type of treatment. We also introduce a new method to analytically calculate the selection gradient in periodic environments, which allows our predictions to be interpreted using the concept of reproductive value, and can be applied more generally to analyse eco-evolutionary dynamics in class-structured populations and fluctuating environments.

Eradicating poliomyelitis: India's journey from hyperendemic to polio-free status

India's success in eliminating wild polioviruses (WPVs) has been acclaimed globally. Since the last case on January 13, 2011 success has been sustained for two years. By early 2014 India could be certified free of WPV transmission, if no indigenous transmission occurs, the chances of which is considered zero. Until early 1990s India was hyperendemic for polio, with an average of 500 to 1000 children getting paralysed daily. In spite of introducing trivalent oral poliovirus vaccine (tOPV) in the Expanded Programme on Immunization (EPI) in 1979, the burden of polio did not fall below that of the pre-EPI era for a decade. One of the main reasons was the low vaccine efficacy (VE) of tOPV against WPV types 1 and 3. The VE of tOPV was highest for type 2 and WPV type 2 was eliminated in 1999 itself as the average per-capita vaccine coverage reached 6. The VE against types 1 and 3 was the lowest in Uttar Pradesh and Bihar, where the force of transmission of WPVs was maximum on account of the highest infant-population density. Transmission was finally interrupted with sustained and extraordinary efforts. During the years since 2004 annual pulse polio vaccination campaigns were conducted 10 times each year, virtually every child was tracked and vaccinated - including in all transit points and transport vehicles, monovalent OPV types 1 and 3 were licensed and applied in titrated campaigns according to WPV epidemiology and bivalent OPV (bOPV, with both types 1 and 3) was developed and judiciously deployed. Elimination of WPVs with OPV is only phase 1 of polio eradication. India is poised to progress to phase 2, with introduction of inactivated poliovirus vaccine (IPV), switch from tOPV to bOPV and final elimination of all vaccine-related and vaccine-derived polioviruses. True polio eradication demands zero incidence of poliovirus infection, wild and vaccine.

Seasonal infectious disease epidemiology

Seasonal change in the incidence of infectious diseases is a common phenomenon in both temperate and tropical climates. However, the mechanisms responsible for seasonal disease incidence, and the epidemiological consequences of seasonality, are poorly understood with rare exception. Standard epidemiological theory and concepts such as the basic reproductive number R0 no longer apply, and the implications for interventions that themselves may be periodic, such as pulse vaccination, have not been formally examined. This paper examines the causes and consequences of seasonality, and in so doing derives several new results concerning vaccination strategy and the interpretation of disease outbreak data. It begins with a brief review of published scientific studies in support of different causes of seasonality in infectious diseases of humans, identifying four principal mechanisms and their association with different routes of transmission. It then describes the consequences of seasonality for R0, disease outbreaks, endemic dynamics and persistence. Finally, a mathematical analysis of routine and pulse vaccination programmes for seasonal infections is presented. The synthesis of seasonal infectious disease epidemiology attempted by this paper highlights the need for further empirical and theoretical work.

Herd immunity and herd effect: new insights and definitions

The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed. It is defined as: "the reduction of infection or disease in the unimmunised segment as a result of immunising a proportion of the population". Herd immunity can be measured by testing a sample of the population for the presence of the chosen immune parameter. Herd effect can be measured by quantifying the decline in incidence in the unimmunised segment of a population in which an immunisation programme is instituted. Herd immunity applies to immunisation or infection, human to human transmitted or otherwise. On the other hand, herd effect applies to immunisation or other health interventions which reduce the probability of transmission, confined to infections transmitted human to human, directly or via vector. The induced herd immunity of a given vaccine exhibits geographic variation as it depends upon coverage and efficacy of the vaccine, both of which can vary geographically. Herd effect is determined by herd immunity as well as the force of transmission of the corresponding infection. Clear understanding of these phenomena and their relationships will help improve the design of effective and efficient immunisation programmes aimed at control, elimination or eradication of vaccine preventable infectious diseases.

The use of mass campaigns in the expanded program on immunization: a review of reported advantages and disadvantages

The use of mass immunization campaigns (MICs) has been and remains controversial. To evaluate these campaigns, the authors review the literature relating to their effectiveness, sustainability, and cost-effectiveness in controlling diseases and raising immunization coverage levels, and their impact on the subsequent development of routine immunization services. Well-conducted campaigns have increased vaccine coverage levels and decreased disease morbidity and mortality. Their use in the Americas has been associated with the apparent elimination of poliomyelitis. However, unless health care infrastructure is improved, or campaigns are repeated, gains in coverage levels may not be sustained. Studies suggest that MICs are often not as cost-effective for raising coverage as the delivery of vaccines through routine services, but the use of coverage as the only outcome measure is questionable. Mass immunization campaigns can increase awareness of vaccination and may be appropriate in situations where new programs are to be initiated, in refugee situations where people congregate into areas with little infrastructure, and in disease eradication efforts when specific time goals are set. Little information is available on whether MICs strengthen or interfere with the development of routine services. To be successful, MICs require a well-coordinated and planned effort on the part of national authorities with the identification of specific goals, intensive social promotion, and strong management. In addition, research is needed to clarify how MICs should be evaluated.

Polio eradication--target 2000

Poliovirus an enterovirus is of 3 types (1, 2 & 3). The 1 poliovirus is most often the cause of paralysis. Poliomyelitis can be eradicated from human soil as man is the only reservoir of this infection and effective vaccines are also available for its control. Inactivated poliovirus vaccine (IPV) confers mainly the humoral immunity in comparison to oral polio vaccine (OPV) which gives the intestinal immunity as well. OPV has been recommended by the WHO as the vaccine of choice for global eradication of polio because of its superior ability to inhibit spread of wild polio virus, low cost and its ease of administration. 70-90% of polio cases occur in children less than 3 year of age. Each paralytic case which is the tip of an iceberg probably represents 100 to 1,000 infected persons in the community. The incidence of poliomyelitis is on the decline with 145 of 213 countries today report 0 case of polio. Central Africa & South Asia are the principal reservoirs of wild poliovirus with nearly two thirds of cases being reported from Indian subcontinent. Components of eradication strategy are: sustained high levels of immunisation, annual mass vaccination campaigns of OPV to all children under 5 years of age, establishment of extremely sensitive surveillance systems and targeted immunisation to areas and populations where poliovirus transmission is likely to persist. The task of global eradication of poliomyelitis is uphill but well within our reach. A strong will and political commitment by the Government of India is leading the nation to the goal of polio-free world by the year 2,000.

Vaccine-preventable disease and immunization in the developing world

Vaccines have given health care providers control over a substantial portion of the morbidity and mortality in the developing world. Global efforts have immunized two-thirds of the world's children with DTP and polio vaccines; 72% have received BCG and 59% measles vaccine; but only 29% of pregnant women have received two doses of tetanus toxoid. In addition, vaccines against yellow fever, Japanese encephalitis, hepatitis B, rubella, and mumps and meningococcal polysaccharide vaccine are being used in specific regions of the world. New vaccine candidates will enhance the vaccine armamentarium over the next decade to include the causes of pneumonia, diarrhea, and meningitis: Haemophilus influenzae type b, pneumococcal and meningococcal protein conjugate vaccines, typhoid and rotavirus vaccine. Genetically engineered vaccine vehicles, genetic reassortants, and genetic deletions are being investigated as new vaccine candidates.

Poliovirus vaccines. A continuing challenge

The control of poliomyelitis remains a provocative challenge. Alternative vaccination schedules, continuing research toward better vaccines, and ongoing international scientific, epidemiologic, and economic collaboration may make it possible to provide effective immunization for all children of the world and eventually may eradicate poliomyelitis worldwide, a goal set forward by the Expanded Programme on Immunization of the World Health Organization.

Vaccination strategies in developing countries

By 1990 it is hoped that all of the world's infants will have access to immunization services and that these services will then continue indefinitely. The link between people and health services, including immunization, can only be forged and maintained by an effective system of delivery and support to all health workers. A careful choice of strategies for this delivery system and an understanding of local cultural attitudes and behaviour is vital if this link is to be effective. Health workers will have to be trained and then supported in the field by regular contact with their supervisors. They will also need continuous, reliable, predictable and adequate supplies of equipment, drugs, vaccines, fuel and money, including salaries. Immunization is cost effective as a health intervention, but an effective programme of immunization can contribute much more than just vaccines, if it is developed in the context of primary health care (PHC) as originally proposed in 1978 at the conference in Alma Ata.

Paralytic poliomyelitis in Italy (1981-85)

Fifteen cases of presumptive poliomyelitis occurring in Italy between 1981-85 were studied in order to differentiate between paralysis caused by poliovirus and that of different etiology. Out of seven confirmed cases three were "temporally associated with vaccination". Three aspects are discussed: the need for a careful differential diagnosis of paralytic cases; the over concern about the problem of vaccine-associated cases: the risk connected with re-importation of wild poliovirus strains.