IMPORTANCE OF URBAN MEASLES TRANSMISSION/ MEASLES ELIMINATION, GERMANY/ MEASLES FATALITY IN NEPAL

Wednesday, 15th of April 2009 Print
                                   CHILD SURVIVAL UPDATE 22/2009: THREE ON MEASLES
 
Measles is on everyone's agenda these days, so I am sending out three items on the subject.
 
With apologies to Oscar Wilde, I am leading off this update with an unpublished report of mine on 'The Importance of Being Urban.'
 
Two country reports follow, one each from Germany and Nepal.
 
1)  THE IMPORTANCE OF BEING URBAN
 

The Importance of Being Urban

 

 

EPIDEMIOLOGY

 

Writing in 1979, Yorke and colleagues set down the requirements for perpetuation and eradication of measles in populations.[1]

‘Perpetuation of a virus in a population is distinct from the ability to persist in a cell culture or individual host. Parameters which determine perpetuation include: 1) the size of the population; 2) the turnover of the population; 3) the proportion of immunes in the population; 4) the transmissibility of the infection; and 5) the generation time between sequential infections. These parameters may be grouped into two composite factors which most directly affect transmission dynamics and perpetuation: (a) population turnover per generation period, and (b) transmissibility or the fraction of susceptibles infected per existing infection. Perpetuation in small populations usually requires either the ability to persist in individuals or rapid population turnover. Conversely, human viruses which initiate only acute infections require larger populations to persist.’

 

Measles epidemiology has for many decades recognized the importance of critical community size for perpetuation of measles transmission. Different authors have placed the CCS at 250-500,000. [2]  The most recent published discussion on the subject is by Andrew Conlan and Bryan Grenfell, ‘Seasonality and the Persistence and Invasion of Measles,’ in the Proceedings of the Royal Society B, at http://rspb.royalsocietypublishing.org/content/274/1614/1133.full

 

One point made by Conlan and Grenfell is of direct importance to African cities.

 

‘In some populations with high transmission and birth rates, the lengthening of epidemic periods after vaccination could significantly raise the CCS in high birth rate countries, with significant consequences for the spatial spread of measles between isolated settlements. The travelling waves of measles observed in England and Wales (Cliff et al. 1993; Grenfell et al. 2001) were the result of infective ‘sparks’ from persistent urban hubs (above the CCS) restarting epidemics in smaller settlements after epidemic fade-out.’

 

The urban hubs of measles transmission are not limited to the better documented cases of England and Wales. Here is the CDC report of Kenya’s most recent outbreak.

 

‘In September 2005, a cluster of laboratory-confirmed measles cases was reported from a predominantly Somali immigrant community in Nairobi. During September 2005--May 2007, this outbreak grew to a total of 2,544 confirmed measles cases reported from 71 (91%) of the 78 districts, with peak monthly totals of 375 and 332 confirmed cases reported in April and August 2006, respectively. Viruses were isolated from specimens collected from approximately 80 persons and identified as genotype B3, with one additional virus from Rift Valley Province identified as D4.

‘Of the 2,544 confirmed outbreak cases, 944 (37%) were in persons aged 9--59 months, 491 (19%) were in persons aged 5--14 years, and 658 (26%) were in persons aged >15 years. A history of measles vaccination was provided by 466 (18%) of the patients, including 220 (23%) of the 944 children aged 9--59 months and 95 (26%) of the 366 children aged 5--9 years. [3]

In terms of practice, CCS means that larger population units, with larger populations of susceptibles, are better able to perpetuate measles transmission than rural towns and villages. Moreover, any urban transportation link provides the urban infected with the means of spreading infection upcountry. In Kenya’s most recent outbreak, transmission started in Nairobi and spread from there to 78 districts of the country.

 

 

SOCIOLOGY

 

The social factors that bind many villages together tend to break down in large towns and cities, as ethnolinguistic groups mix with each other. The village headman, an authority figure in many Asian and African societies, is sometimes absent from the city.

 

In the Uganda vaccination campaign of March 2009, the overall coverage, including Kampala and 28 rural districts, was 104 percent; Kampala had 86 percent coverage (unpublished documents, Uganda EPI).  Preregistration in Kampala was under 20 percent, the second lowest in the country. This reflects the weak social structures in urban areas.

 

The best way to assure high urban performance is to use those channels (typically, churches, mosques, and FM radio), in addition to the community leaders. Independent monitoring is a useful check on the accuracy of administrative coverage. Typically, administrative coverage, sometimes exceeding 100 percent, will be >15 percent higher than coverage as figured by independent monitors.

 

 



[1] Yorke et al., ‘Seasonality and the requirements for perpetuation and eradication of viruses in populations,’ Amer J Epid, Feb 1979, 103ff.

 

[2] ‘The development of this dynamic understanding for industrialized countries has benefited from rich databases

of measles incidence from the UK, the USA and elsewhere (Cliff et al. 1993; Grenfell et al. 1995; Ferguson et al.

1996). The resulting theoretical models have been used to aid policy and predict the efficacy of vaccination

campaigns (McLean & Anderson 1987b; Babad et al. 1994; Edmunds et al. 2000; Wallinga et al. 2001). The

combination of data and models also allowed Bartlett, Black and others to characterize a CCS of 250–500 000

for urban communities (Bartlett 1957; Black 1966; Keeling & Grenfell 1997). The epidemic dynamics and

seasonal forcing, spatial heterogeneity (Bolker & Grenfell 1995; Lloyd & May 1995; Earn et al. 1998; Grenfell et al. 2001; Xia et al. 2004) and the details of the infection and recovery process (Keeling & Grenfell 1997, 2002; Lloyd 2001b) have been shown to be important in understanding the level of the CCS.’ Conlan and Grenfell, 2007.

[3] ‘Progress in Measles Control, Kenya, 2002-2007,’ Morbidity and Mortality Weekly Report, 21 September 2007, consulted at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5637a5.htm

 
2) GERMANY

As a member state of WHO's EURO region, Germany is committed to time limited eradication. In Germany, as elsewhere, rises in child vaccination have led to a shift in age distribution of measles cases towards older age groups. If this issue is not settled, early regional elimination from EURO is unlikely.

Wichmann and colleagues conclude

       'An accumulation of non-immune individuals led to this outbreak. The shift in age distribution has implications for the effectiveness of measles control and the elimination strategy in place. Immediate nationwide school-based catch-up vaccination campaigns targeting older age groups are needed to close  critical immunity gaps. Otherwise, the elimination of measles in Germany and thus in Europe by 2010 will not be feasible.'

 Full text is at http://www.who.int/bulletin/volumes/87/2/07-050187.pdf

3) NATIONWIDE CFR FOR MEASLES, NEPAL

This Nepal team, with staff from the government and CDC, set itself the ambitious task of estimating the national case fatality rate for the entire, somewhat heterogeneous, country. Here are their findings.

       'We identified 4657 measles cases and 64 deaths in the study period and area. This yielded a total of about 82 000
       cases and 900 deaths for all outbreaks in 2004 and a national CFR of 1.1% (95% confidence interval,  CI: 0.5-2.3).
       CFR ranged from 0.1% in the eastern region to 3.4% in the mid-western region and was highest  in politically insecure
       areas, in the Ganges plains and among cases < 5 years of age.'
       'Vitamin A treatment and measles immunization were protective. Most deaths occurred during the first week  
       of illness.'
 
It is remarkable that a country with serious security problems and uneven health service coverage should manage to get vitamin A treatment to almost half its measles cases. This shows how a strong IMCI activity can complement measles vaccination in reducing measles mortality.
 
Full text is at http://www.who.int/bulletin/publish_ahead_of_print/en/index.html
 

BD

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