Fri, 16 Feb 2018 08:42:21 +0000
Prof Eustarckio Kazonga
IMMUNISATION is the process in which a person is made resistant to an infectious disease through vaccination.
Both adults and children are protected through immunisation against many serious infectious diseases.
An immunisation programme, through vaccination, aims at protecting the individual and to prevent the spread of such diseases in the wider population.
A vaccine stimulates the immune system to protect a person against subsequent infection or disease.
Vaccination coverage information is useful for identifying areas and groups with lower vaccination coverage so that responsible health officials and other stakeholders can take action to help improve vaccination coverage and protect everyone from vaccine-preventable diseases.
This article is, therefore, aimed at illustrating how statistics is used in an immunisation programme.
Some of the concepts in immunisation that make use of statistics are: immunisation uptake, vaccine coverage and uptake.
Oral Cholera Vaccine (OCV)
The Minister of Health, Dr. Chitalu Chilufya, officially flagged off the oral cholera vaccine on 10th January 2018. Dr. Chilufya and the Deputy Secretary to the Cabinet were seen taking the cholera vaccine during the official launch of the vaccine programme.
In spite of the myths surrounding the vaccine, residents in cholera epicentres of Kanyama, Chawama, Matero and Mandevu turned up in numbers to receive OCV.
Being a two-dose vaccine, it is supposed to be taken twice. Statistics can be used in the administration and management of OCV through the calculation of vaccine coverage.
Immunisation uptake, sometimes referred to as coverage, refers to the proportion of the eligible population who have received the recommended doses of the relevant vaccines.
Monitoring the proportion of the eligible population vaccinated is a key measure of an immunisation programme.
For example, the European Region of the World Health Organisation (WHO) recommends that on a national basis at least 95 percent of children are immunised against diseases preventable by immunisation and targeted for elimination or control of diseases such diphtheria, tetanus, pertussis, polio, Hib, measles, mumps and rubella (WHO, 1999).
According to the Zambia Demographic and Health Survey (ZDHS) (2013-14), 58 percent of children received all of the basic vaccinations by the age of 12 months.
Overall, 68 percent of children aged 12-23 months were fully immunised by the time of the survey.
With regard to specific vaccines, 95 percent of children received the BCG immunisation, and 85 percent were immunised against measles.
Coverage of the first dose of the DTP and polio vaccines was relatively high (96 percent each).
However, only 86 percent and 78 percent of these children, respectively, went on to receive the third doses of these vaccines, contributing to respective dropout rates of 11 percent and 19 percent between the first and third doses.
The survey findings showed that 2 percent of children aged 12-23 months did not receive any vaccine at all.
It is further reported that 76 percent of urban children were fully immunised, as compared with 65 percent of rural children.
Full immunisation coverage ranged from 60 percent in Luapula province to 81 percent in Copperbelt province. Sixty-eight percent of children age 12-23 months are fully immunised (ZDHS, 2015-14).
These statistics can be compared to Zimbabwe’s Demographic and Health Survey 2015 which recorded seventy-six percent of children aged 12-23 months receiving all basic vaccinations at the time of the survey.
This was an increase from 65 percent recorded in 2010-11 survey.
In particular, 90 percent received the BCG vaccine, 83 percent received three doses of DPT or pentavalent vaccine, 82 percent received three doses of polio vaccine, and 82 percent received one dose of the measles vaccine.
Eighty-two percent of children completed three doses of the pneumococcal vaccine and 50 percent completed two doses of the rotavirus vaccine.
Ten percent of children aged 12-23 months did not receive any vaccinations. The coverage of the first dose of pentavalent and polio vaccines is very high (90 percent each).
However, 83 percent of children received the third dose of pentavalent, while 82 percent received the third dose of the polio vaccine.
This represents a dropout rate between the first and third dose of 7 percentage points for the pentavalent vaccine and 8 percentage points for the polio vaccine (Zimbabwe DHS, 2015:166-167).
2007 Zambia Demographic and Health Survey
The 2013-14 ZDHS immunisation statistics can be compared to the previous one of 2007.
The survey findings showed that vaccination coverage was higher in urban areas than in rural areas.
For example, 95 percent of urban children received a BCG vaccination, compared with 91 percent of rural children; 89 percent of urban children received all three DPT/DPT-HepB-Hib vaccinations, compared with 76 percent of their rural counterparts. Similarly, polio 3 and measles coverage is higher among children in urban areas (81 and 89 percent, respectively) than those in rural areas (76 and 84 percent, respectively).
Children in rural areas were twice as likely as those in urban areas to have not received any vaccinations (3 and 6 percent, respectively).
There were provincial variations in coverage for all vaccinations.
Overall, BCG coverage was above 90 percent for all provinces except Luapula (84 percent) and Northern (82 percent).
Polio 3 coverage was 78 percent or higher in all provinces except Luapula (71 percent), Northern (69 percent) and North-Western (59 percent) provinces. Measles coverage is above 75 percent in all provinces except Northern (71 percent) (ZDHS, 2007:141-143).
The proportion of children who received all basic vaccinations increased with mother’s level of education; 60 percent of children whose mothers had no education were fully immunised, compared with 75 percent of children of mothers with secondary education, and 77 percent of children of mothers with more than a secondary education. Children in the lowest and highest wealth quintiles are most likely to be fully vaccinated (71 and 78 percent, respectively).
Uptake rates, sometimes referred to as coverage, relate to the number of people immunised against a particular disease as a proportion of the people in the population eligible to have received the appropriate vaccine. Uptake is calculated as follows:
(Total number of eligible people immunised)/(Total number of eligible people in the population) x 100
Standard definitions used for adult immunisation statistics differ slightly from those used for childhood immunisations, specifically using a different definition for coverage and uptake: (i) Vaccine coverage: The proportion of those eligible who have ever received the vaccine (ii) Vaccine uptake: The proportion of those eligible who received their vaccine during the specified period (WHO).
Children Immunisation Statistics
Childhood immunisation indicators include BCG immunisation coverage among one-year-olds, measles immunisation coverage among one-year-olds, polio immunisation coverage among one-year-olds, DTP3 immunisation coverage among one-year-olds and full immunisation coverage among one-year-olds.
Note that the reference age group used in the denominator (12–23 months) was adjusted to align with alternative immunisation schedules adopted in certain countries (18–29 months or 15–26 months).
The World Health Organisation defines a number of children immunisation statistics as follows:
(a) BCG immunisation coverage among one-year-olds is defined as the percentage of one-year-olds who have received one dose of Bacile Calmette-Guérin (BCG) vaccine;
(b) Measles immunisation coverage among one-year-olds is defined as the percentage of one-year-olds who have received at least one dose of measles-containing vaccine;
(c) Polio immunisation coverage among one-year-olds is defined as the percentage of one-year-olds who have received three doses of polio vaccine;
(d) DTP3 immunisation coverage among one-year-olds is defined as the percentage of one-year-olds who have received three doses of the combined diphtheria, tetanus toxoid and pertussis (DTP3) vaccine; and
(e) Full immunisation coverage among one-year-olds is defined as the percentage of one-year-olds who have received one dose of BCG vaccine, three doses of polio vaccine, three doses of DTP3 vaccine, and one dose of measles vaccine.
These statistics require appropriate data to be collected, analysed and interpreted for monitoring and evaluation, making comparisons and rational decision-making.
Universal immunisation of children against the six vaccine-preventable diseases i.e. tuberculosis, diphtheria, whooping cough, tetanus, polio, and measles is very important in reducing infant and child mortality.
Publicly available data from the Demographic and Health Surveys (DHS) can be used to compare progress in time periods within the country but also across countries.
Specifically, childhood immunisation statistics can be used to monitor the performance of the national childhood immunisation programme including targets.
From the public health point of view, if immunisation rates decrease, it makes the possibility of disease transmission more likely.
Immunisation statistics can, therefore, be used to inform vaccine policy decision making and planning in the co-ordination and strengthening of health protection in the country.
Furthermore, it has been illustrated that immunisation statistics are used to inform the development and evaluation of government policy on immunisation and to assess the delivery of different immunisations in the national programme.