Official country estimates MCV1 Official country estimates MCV2 WHO estimates MCV1 WHO estimates MCV2
56 Which of these two data sets is more accurate is not clear, yet they both show a similar picture:
vaccination rates are generally below 90%. Rates increased dramatically, presumably because of SIAs, in the aftermath of measles epidemics. As previously stated, outbreaks of measles occur readily in populations where more than 10% is susceptible to measles infection, as is the case in South Africa. Generally the official South African estimates seem more optimistic with regards to vaccine coverage than the WHO estimates. Yet, even if we accept the official South African estimates we can see that the vaccination rates in South Africa are too low to achieve elimination of measles transmission.
To achieve elimination of measles within a country and to sustain elimination, the WHO recommends vaccination with two doses sustained at levels upwards of 95% (Shibeshi et al.
2014). The data from the US and Canada confirms the effectiveness and necessity of such sustained levels of vaccination to eliminate measles.
It is therefore clear that current vaccination levels in South Africa are inadequate to attain elimination of measles. If vaccination levels are maintained at current estimated levels, South Africa will remain susceptible to large epidemics. After an epidemic there appears to be increased vigilance to ensure adequate vaccination and an increase in the coverage of routine vaccinations as well as SIAs. The subsequent rise in vaccination coverage combined with increased levels of natural immunity acquired from measles infection lead to decreased susceptibility to measles outbreaks in the population. During these times population immunity to measles is high and the number of measles cases fall. If vaccination rates remain below the elimination threshold, the number of susceptible persons in the population accumulates over time. It then is inevitable that at a certain point the susceptibility within the population will again lead to a large epidemic.
The HIV epidemic does not seem to have influenced measles spread or the epidemiological pattern of measles in South Africa substantially (Helfand et al. 2005; Shibeshi et al. 2014). It is so that there is an increase in the proportion of the population susceptible to measles because of a higher rate of vaccine failure among HIV positive children. This is estimated to be a 2-3% increase in the susceptible pool of each new birth cohort. This increase in susceptibility is largely neutralized by the higher all-cause death rates among HIV positive children, so that the overall susceptibility to measles within the population does not seem to be substantially increased by the HIV pandemic (Helfand et al. 2005). The experience with measles spread during the HIV pandemic in South Africa confirms that the impact of the pandemic has been minimal (Helfand et al. 2005; Shibeshi et al. 2014). Rather, it is that absence of sufficient vaccination coverage that is responsible for the measles epidemics in South Africa. It has been shown that the most significant factor leading to increased susceptibility of children in South Africa to measles infection is lack of a second measles vaccination opportunity (Helfand et al. 2005).
Even though HIV has not affected the pattern of measles spread significantly, individual HIV affected children are more vulnerable to measles illness and its complications (Helfand et al.
2005; Shibeshi et al. 2014). Children born to HIV positive mothers have less maternal protective antibodies, and such children are therefore more susceptible to measles infections (Helfand et
57 al. 2005; Shibeshi et al. 2014). It also appears that HIV positive children have a lesser response to measles vaccine and have a decline in vaccine protection more quickly (Helfand et al. 2005;
Shibeshi et al. 2014). Therefore, although HIV has not increased the spread of measles, children who are affected by HIV are more prone to becoming infected by measles and therefore are more vulnerable than the general population. It is different for adults or adolescents who become infected with HIV after getting measles vaccine when they were not HIV positive; these persons seem to keep their immunity to measles (Helfand et al. 2005). It is specifically HIV infected/affected children that are at higher risk, and it makes sense to offer such children measles vaccination at a younger age. For example, there is a WHO recommendation adding a measles vaccine at 6 months in addition to the usual 9 month/18 month schedule for infants who are born in areas with very high HIV and measles levels (Shibeshi et al. 2014).
It seems clear given the epidemiological data that simple steps to curb measles spread and epidemics in South Africa would include high rate of routine coverage with MCV1 and MCV2, while ensuring numerous strategic SIA outreaches to reach those who do not have sufficient access to routine vaccine coverage.
Europe
Fourteen years after measles was eliminated in the United Kingdom, measles was once again declared to be endemic in that country in June 2008 (CDC 2008; Euro Surveillance Editorial Team 2008). The statement from the Health Protection Agency at that time was that “the number of children susceptible to measles is now sufficient to support the continuous spread of measles”
(Euro Surveillance Editorial Team 2008). The reason for this was that vaccination rates had dropped to 80-85% for a number of years, so that the susceptible portion of the population increased each year (CDC 2008).
The case of the United Kingdom provides evidence that even if measles is eliminated in a country, it is necessary to maintain high levels of immunity against measles in the population through vaccination to prevent measles from returning to a pattern of continuous spread. When vaccination rates fall in countries where measles has been eliminated, every year with every birth cohort the pool of people susceptible to measles infection grows within that country. As the United Kingdom example bears out, the country then becomes vulnerable to the return of measles as an endemic illness with continuous spread within the country. This is no surprise;
measles is highly contagious and cases can be imported quite easily from countries where measles still occur endemically to countries where measles had been eliminated.
Table 1.1 shows the number of measles cases per year in recent years in the United Kingdom, as well as vaccination rates. Just recently vaccination rates seem to have returned to higher levels in the United Kingdom, showing rates of 90% or higher since 2011.
A recent outbreak in the Netherlands provides interesting insights with regards to the spread of measles in a country that has a high vaccination rate. In the Netherlands measles vaccination
58 rates are typically over 95%. Yet, in 2013 the Netherlands experienced a measles outbreak affecting well over a 1,000 cases. In fact, in table 1.1 it is shown that the reported number of measles cases in the Netherlands for 2013 was 2,653 which is a marked increase from the previous year’s 10 cases. This reflects the magnitude of the outbreak in 2013.
One article reported statistics from this 2013 epidemic as on 28 August 2013 and discussed the implications of this outbreak (Knol et al. 2013). At that stage there had been 1,226 reported measles cases with 82 hospitalizations. There was a strong correlation with being unvaccinated, with 96.5% of the cases occurring in unvaccinated people. There was also a strong correlation with being an orthodox Protestant, with 91.7% of affected people being orthodox Protestants.
The authors also estimate that the official statistics (reported measles cases) represent an underestimation of the true magnitude of the outbreak, since experience with previous measles outbreaks in the Netherlands have shown that many people infected by measles do not seek physician treatment. If the same level of under-reporting existed in the 2013 outbreak as in previous outbreaks, the number of measles cases in the 2013 outbreak would actually be around 13,000. The author also argues that although most cases had been Protestants in the 2013 outbreak, it is likely that this epidemic will eventually spread to non-Protestants as well. Only 15% of vaccine-refusers in the Netherlands are Protestants. The Protestant vaccine-refusers tend to live close to each other in congregated communities, making measles spread within these communities easy. Non-protestant vaccine-refusers tend to be more widely distributed and interspersed within the highly vaccinated population, so that they are protected by herd-immunity. Herd-immunity will be discussed in a separate section below.
From the Netherlands outbreak we can see that even if a country has very high levels of vaccination, pockets of susceptible populations can still exist within the country due to the congregation of vaccine-refusers in communities. Where susceptible persons live in close proximity or congregate together, measles outbreaks are likely. Where susceptible persons are spread out within a highly vaccinated population, they seem to be relatively more protected against measles by virtue of the high vaccination rate in the general population. This is similar to the experience in the US and in Canada, where high vaccination rates exist but where pockets of susceptible populations exist within the country. When measles is imported from elsewhere, these pockets of susceptible populations are at high risk of experiencing measles outbreaks.
Global
The World Health Organization is engaged in ongoing efforts to increase vaccination coverage globally and to decrease the global burden of measles (CDC 2013c). Their strategy focuses on increasing MCV1 and MVC2 coverage world-wide through various monitoring and intervention measures. These measures have been met by some success; estimates of global MCV1 coverage has increased from 72% in the year 2000 to 84% in the year 2011, and the number of countries providing MCV2 has increased from 97 (50%) in 2000 to 141 (73%) in 2011. Correspondingly, the estimated number of measles cases per population has decreased in this time period as did the
59 estimated number of measles deaths. In 2000, the estimated global measles cases per 1 million population was 146 per year, while in 2011 this number dropped to 52 per year. In 2000 the estimated number of global measles deaths were 542,000 while in 2011 the estimated number of global measles deaths were 158,000 (CDC 2013c).
According to figure 1.3, compiled from WHO statistics, the estimated number of measles deaths globally was 564,200 in 2000 and 122,000 in 2012. The estimates of global mortality for 2000 vary very slightly between the two sources, within the range of being acceptable. The estimate in figure 1.3 (564,200) is the latest estimate by the WHO of measles deaths in 2000. Both sets of estimates show a similar decline in measles case rates as well as global mortality rates over the corresponding time period.
In 2010-2011 there was an increase in the global measles incidence rate with many countries experiencing an increase in reported measles cases. There were numerous countries that experienced substantial measles outbreaks in 2011: the DRC, India, Indonesia, Nigeria, Somalia, France, Zambia, Chad, Philippines, Sudan, Italy, Pakistan, Romania, Spain, Uganda, Ethiopia and Afghanistan. Investigations into some outbreaks in this time period suggested the main cause of these outbreaks was “persistent gaps in immunization coverage, despite overall increased measles vaccine coverage” (CDC 2013c).
It should also be pointed out that the estimated global measles mortality is based on a WHO model that is dependent on reported vaccination rates and reported measles cases. This explains the slight difference between the two estimates highlighted earlier; as updated vaccine and measles case reporting becomes available, the WHO updates the estimates according to their model (CDC 2013c).
Some of the countries who have the highest disease burden do not have the ability to report exact number of cases accurately (CDC 2013c). Also, it seems reasonable given the socio-political realities in most of the countries with high measles burden that the actual number of cases are underreported while vaccination coverage may be overestimated. Yet, it does seem that there has been a marked decline in global measles burden that is attributable to an increase in global measles vaccine coverage.
Even though this is good news, it is clear that measles still presents a worldwide health challenge and that measles is responsible for substantial global burden of disease. It also seems that there are many countries experiencing outbreaks and continuous measles spread from where measles can be imported to countries where measles have been eliminated. Thus, although progress has been made the numbers bear out that measles is far from a vanquished foe when speaking in global terms.
60