Emergencies preparedness, response

Circulating vaccine-derived poliovirus – Lao People’s Democratic Republic

Disease outbreak news
26 November 2015

Two additional cases of type 1 vaccine-derived poliovirus (VDPV1) have been reported from Lao People’s Democratic Republic (PDR), bringing the total number of cases in this outbreak to three.

Between 6 and 8 November 2015, the National IHR Focal Point of the Lao People’s Democratic Republic (PDR) notified WHO of 2 confirmed VDPV1 cases. Furthermore, circulating VDPV1 (cVDPV1) has also been isolated from the stools of 12 healthy contacts. All these contacts live in the same village, Bolikhan district (Bolikhamxay Province).

Details of the cases

The first of the new cases is a 15-year-old male. He comes from the same village as the first confirmed cVDPV1 case. He had no history of receiving oral polio vaccine (OPV). Considering that a nucleotide divergence of around 2.6% was detected between the isolate of the 15 year-old VDPV1 case and Sabin 1 strain, and considering that 3 close contacts tested positive for cVDPV1, the 15-year-old case was classified as a cVDPV1.

The second case is a 4-year-old male with onset of fever on 28 September and lower limb weakness on 29 September. On 1 October, the patient was admitted to the intensive care unit of Bolikhamxay Provincial Hospital, where he passed away on 2 October, before stool specimens could be collected. The patient was classified as an acute flaccid paralysis (AFP) case following retrospective review conducted at Bolikhamxay Provincial Hospital on 19 October. He had no history of receiving OPV. On 20 and 21 October, stool specimens were collected from 4 of his contacts, including household contacts and contacts from neighboring households; on 6 November, three of these contacts tested positive for cVDPV1. According to WHO criteria, the 4 year-old AFP case was therefore classified as a confirmed cVDPV1 case.

The National IHR Focal Point also provided an update on the investigation of contacts of the first confirmed cVDPV1 case (see DON published on 10 October). Stool samples from contacts were collected and cVDPV1 was isolated in 6 of them.

Previous to this outbreak, Bolikhan district had chronically low immunization rates: reported infant coverage with 3 doses of OPV was 40% to 66% between 2009 and 2014. Lao PDR’s last case of indigenous wild poliovirus occurred in 1993.

Public health response

Since the detection of the first confirmed cVDPV1 in Lao PDR, outbreak response activities have been conducted in three provinces, including the affected province (Bolikhamxay) and neighbouring provinces (Xaisounboun and Xiengkhuang). The national emergency operations centre has been activated to coordinate response efforts and a polio outbreak response plan was drafted. Enhanced surveillance is occurring throughout the country including daily zero-reporting of AFP cases. Active case finding is ongoing in the three provinces, including retrospective review of hospital and health centre records.

Six rounds with trivalent OPV vaccine have been planned from October 2015 to March 2016 (4 sub-national and 2 national) with ~ 8.6 million doses to be administered to children younger than 15 years. This age range was determined by the age distribution of the cases and their contacts. The first round of supplementary immunization activity (SIA) with OPV vaccine was completed in October in Bolikhamxay, Xaisounboun and Xiengkhuang provinces. Monitoring of October OPV SIA has occurred to identify areas with missed children and plan for mop-up activities. It is planned that this will continue during the next rounds to identify villages that require mop-up. Independent monitors were recruited to assess the quality of the campaigns.

To ensure the success of SIAs, emergency risk communications and social mobilization activities, including training of mobilizers and information sessions to build trust and address barriers to immunization, are being conducted. Key messages have been developed for radio and loudspeaker and are being translated to target identified communities.

WHO risk assessment

cVDPVs are rare but well-documented strains of poliovirus mutated from strains in OPV. They can emerge in some populations that are inadequately immunized.

Ending polio for good requires eliminating both wild and vaccine-derived polio, and due to the small risk of cVDPVs, use of OPV must be stopped to secure a lasting polio-free world. OPV will be withdrawn in a phased manner, beginning with the removal of type 2-containing OPV. The switch from trivalent to bivalent OPV, planned in April 2016, will reduce the risk of cVDPV substantially (as 90% of cVDPV is caused by type 2) and sets the stage to eventually stop using OPV altogether and transition to the inactivated polio vaccine (IPV), which cannot cause cVDPV.

Because of relatively limited travel to and from this area and the planned immunization activities, the WHO assesses the risk of international spread of the cVDPV1 from Lao to be low.

WHO advice

It is important that all countries, in particular those with frequent travel and contacts with polio-affected countries and areas, strengthen surveillance for AFP cases in order to rapidly detect any new virus importation and to facilitate a rapid response. Countries, territories and areas should also maintain uniformly high routine immunization coverage at the district level to minimize the consequences of any new virus introduction.

WHO’s International Travel and Health recommends that all travelers to polio-affected areas be fully vaccinated against polio. Residents (and visitors for more than 4 weeks) from infected areas should receive an additional dose of OPV or IPV within 4 weeks to 12 months of travel.