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To the Editor: Since their inception in 1996, National Immunization Days (NIDs) in Myanmar have reportedly provided oral poliovirus vaccine (OPV) to more than 95% of the target population (those younger than 5 years).¹ In conjunction with this, nationwide surveillance for acute flaccid paralysis (AFP) began shortly after the completion of the initial NIDs. In 1999, wild-type poliomyelitis virus was detected for the first time in 4 patients with AFP, aged 8 to 23 months, who lived in a state bordering Bangladesh.^2-3 This might be seen as a sign of improvement in AFP surveillance, which has been a priority in Myanmar, especially in difficult-to-reach parts of the country. However, the overall seroprevalence of poliomyelitis in Myanmar is still unknown.

Methods
We estimated the seroprevalence of poliomyelitis in Yangon, Myanmar, from serum samples obtained between August 1998 and May 1999. Because a randomized serosurvey was not feasible, serum specimens originally drawn for diagnosis of dengue fever and specimens of blood donors were used as a convenience sample for child and adult populations, respectively. We deliberately tested a greater number of samples from younger subjects to allow a more precise comparison between children younger than 10 years and teenagers (who had less frequent opportunities to receive the vaccine in infancy). A total of 514 samples were tested. Of these, 114 (22%) were from subjects aged 4 years and younger, 108 (21%) from those aged 5 to 9 years, 102 (20%) from those aged 10 to 14 years, 100 (19%) from those aged 15 to 19 years, and 30 (6%) each from 3 groups of those aged 20 to 29 years, 30 to 39 years, and 40 years and older. Neutralization tests for poliomyelitis antibodies were carried out at the World Health Organization (WHO)–accredited national poliomyelitis laboratory of Myanmar following the internationally standardized WHO procedures. Seropositivity was defined as an antibody titer of 1:8 or greater.

Results
The results shown in Figure 1 illustrate similar U-shaped seropositivity patterns against the 3 serotypes (type 1, type 2, and type 3) of poliovirus, with the lowest seropositivity rates among teenagers (aged 10-19 years). The lower rates among teenagers were statistically significant (² test, P<.001 for all comparisons) in all 3 serotypes of poliovirus compared with the group younger than 10 years and a combination of all the age groups other than teenagers. The comparison of geometric mean titer of serum neutralizing antibody to any serotype of poliovirus also was statistically significant (t test, P<.001).

View larger version (13K): [in this window] [in a new window] Figure. Poliomyelitis Seropositivity Rate by Age Group Seropositivity was defined as a neutralizing antibody titer of 1:8 or greater.

Comment
This lower rate of immunity among teenagers could be attributed in part to both their less frequent opportunities to receive OPV immunizations in their infancy compared with the current standard⁴ and to a reduced risk of infection with wild-type poliovirus in recent years. In contrast to teenagers, the adult population seemed to maintain higher immunity, apparently owing to natural infections in their infancy.

It is possible that similar patterns of immunity exist in other developing countries. If so, additional strategies to immunize at-risk groups within these populations would seem to be indicated. For instance, a 1-time nationwide vaccination "catch-up" campaign targeting all children younger than 15 years,⁵ such as takes place in a typical measles elimination program, would be a possible step. This sort of modification might be more effective than a mere repetition of current early-childhood NIDs.

AUTHOR INFORMATION
Funding/Support: This study was supported in part by the Japan International Cooperation Agency and a grant from the Japanese Ministry of Health and Welfare.

Acknowledgment: We gratefully acknowledge the contributions made by Yasuo Chiba, MD, Hiroshi Yoshikura, MD, Isao Afita, MD, and Soe Lwin, MD, and his staff at the National Health Laboratory, Yangon, Myanmar.

Kazunobu Kojima, MD, PhD; Shozo Urasawa, MD, PhD
Department of Hygiene
Sapporo Medical University School of Medicine
Sapporo, Japan

Tin Sabai Aung, MMed Sc; Amy Khine, BSc
Virology Section
National Health Laboratory
Yangon, Myanmar

Hlaing Myat Thu, MMedSc
Virology Department
Department of Medical Research
Yangon

1. Progress toward poliomyelitis eradication—Myanmar, 1996-1999. MMWR Morb Mortal Wkly Rep. 1999;48:967-971. PUBMED 2. World Health Organization Regional Office for South-East Asia. SEAR Polio Bulletin. 2000;4:1-2. 3. Department of Health Ministry of Health. Myanmar Polio Newsletter. 2000;1:1-2. 4. Country Report, Union of Myanmar. Fourth meeting of SEAR/EPI Technical Consultative Group (TCG) on vaccine preventable diseases, 1997. Yangon, Myanmar: Ministry of Health; 1998. 5. Progress toward measles elimination—Southern Africa, 1996-1998. MMWR Morb Mortal Wkly Rep. 1999;48:585-589. PUBMED

Letters Section Editors: Stephen J. Lurie, MD, PhD, Senior Editor; Phil B. Fontanarosa, MD, Executive Deputy Editor.

JAMA. 2000;284:2058-2059.

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