Wild poliovirus type 1 (WPV1) - Mozambique - Prevention is better than cure

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23 June 2022

Outbreak at a glance:

On 15 May 2022, a case of wild poliovirus type 1 (WPV1) was reported in Mozambique through the Global Polio Laboratory Network (GPLN). Results of the genomic sequencing analysis indicate that the current WPV1 isolate is genetically linked to a strain detected in Pakistan in 2019 and similar to a case of WPV1 reported in Malawi in February 2022.

As part of response measures following the confirmation of the case in Malawi, two rounds of bivalent oral poliovirus vaccine (bOPV) campaigns have been conducted in the country, with more than 4.5 million children vaccinated.

The risk of international spread, particularly across the South East region of Africa remains high, due to persisting sub-optimal immunity and surveillance gaps, and large-scale population movements.

Description of the outbreak:

On 15 May 2022, the GPLN reported the confirmation of a case of wild poliovirus type 1 (WPV1) in Mozambique. The case is a 12-year-old female with acute flaccid paralysis (AFP), with onset of paralysis on 25 March, from Changara district, Tête province bordering Zimbabwe and Malawi. Two stool specimens were collected for testing on 1 April and 2 April. On 14 May, the samples were confirmed to be WPV1 by National Institute for Communicable Diseases (NICD) in South Africa. The child had previously received three doses of bivalent oral poliovirus vaccine(bOPV) but no inactivated poliovirus vaccine (IPV). Genomic sequencing analysis indicates that the newly confirmed case is linked to a strain that had been circulating in Pakistan in 2019, similar to a case of WPV1 reported in Malawi in February 2022 (For more details on this case, please see the Disease outbreak news published on 3 March 2022). The last indigenous wild poliovirus case in Mozambique was reported in 1993.

Mozambique is also affected by a concurrent outbreak of circulating vaccine-derived poliovirus type 2 (cVDPV2), with seven cases reported in the country since 2021, the most recent on 25 March 2022.

According to the WHO-UNICEF national immunization coverage estimate, the oral poliovirus vaccine third dose (OPV3) and inactivated poliovirus vaccine first dose (IPV1) coverage was 73% and 78% respectively in Mozambique in 2020.

Figure: 1: Countries reporting cases of WPV1 and neighboring countries implementing preparedness plans

Epidemiology of Poliomyelitis

Poliomyelitis (polio) is a highly infectious viral disease that largely affects children under five years of age. The virus is transmitted by person-to-person and spread mainly through the fecal-oral route or, less frequently, by a common vehicle (e.g., contaminated water or food) and multiplies in the intestine, from where it can invade the nervous system and cause paralysis. The virus is shed by infected people (usually children) through faeces, where it can spread quickly, especially in areas with poor hygiene and sanitation systems.

The incubation period is usually 7–10 days but can range from 4–35 days. Up to 90% of those infected are either asymptomatic or experience mild symptoms and the disease usually goes unrecognized. In mildly symptomatic cases, initial symptoms include fever, fatigue, headache, vomiting, stiffness in the neck, and pain in the limbs. These symptoms usually last for 2–10 days and most recovery is complete in almost all cases. However, in the remaining 10% of cases, the virus causes paralysis, usually of the legs, which is most often permanent. Paralysis can occur as rapidly as within a few hours of infection. Of cases with paralysis, 5-10% die when their breathing muscles become immobilized.

Two of the three types of wild poliovirus have been eradicated (WPV2 and WPV3), with ongoing global efforts to eradicate WPV1. Currently, wild poliovirus is endemic in two countries: Pakistan and Afghanistan. The detection of WPV1 outside these two countries where the disease is endemic demonstrates the continuous risk of international spread of the disease until every corner of the world is free of WPV1.

There is no cure for polio; it can only be prevented by immunization.

Public health response

Mozambique has been actively participating in the multi-country emergency outbreak response implemented across the South East Africa region in response to the case of WPV1 reported in Malawi in February 2022, alongside Tanzania, Zambia, and Zimbabwe to reach more than 23 million children across the region. Two rounds of bivalent OPV vaccination campaigns have already been implemented, the most recent at the end of April, with more than 4.5 million children vaccinated in Mozambique. At the same time, the response in the country to the cVDPV2 outbreak is also ongoing.

National and subnational authorities continue to be supported by partners of the Global Polio Eradication Initiative (GPEI), notably by experts of the African Rapid Response Team, the GPLN, UNICEF and local organizations. Surveillance across the sub-region continues to be strengthened.

The detection of the current case underscores the need for a large-scale, rapid, multi-country emergency outbreak response across South East Africa, in line with revised international polio outbreak response SOPs. The foremost priority is to continue to implement the sub-regional emergency response, by continuing to conduct large-scale, rapid and high-quality response campaigns.

WHO risk assessment

Detection of a case of WPV1 in Mozambique, and the second case in the South East region of Africa, confirms ongoing WPV1 transmissions in the sub-region.

WHO considers that there is a continuous high risk of international spread of WPV1, particularly across the South East sub-region of Africa, due to persisting sub-optimal national immunity and surveillance gaps, and large-scale population movements. The risk is further increased due to the decreased immunization rate related to the ongoing COVID-19 pandemic.

The risk of spread associated with the concurrent cVDPV2 outbreak is currently assessed as moderate due to historical and epidemiological evidence suggesting that WPVs have a significantly higher propensity for geographic spread than cVDPVs. However, a comprehensive outbreak response to both strains is urgently being implemented, as both strains have the capacity to cause paralytic disease in children.

WHO advice

It is important that all countries, in particular those with frequent travels and contacts with polio-affected countries and areas, strengthen surveillance for AFP cases and commence planned expansion of environmental surveillance 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 International Travel and Health recommends that all travellers to polio-affected areas should be fully vaccinated against polio. Residents (and visitors for more than four weeks) from affected areas should receive an additional dose of OPV or IPV within four weeks to 12 months of travel.

As per the advice of an Emergency Committee convened under the International Health Regulations (2005), the risk of international spread of poliovirus remains a Public Health Emergency of International Concern (PHEIC). Countries affected by poliovirus transmission are subject to Temporary Recommendations. To comply with the Temporary Recommendations issued under the PHEIC, any country infected by poliovirus should declare the outbreak as a national public health emergency, ensure the vaccination of residents and long-term visitors and restrict at the point of departure travel of individuals, who have not been vaccinated or cannot prove the vaccination status.

The latest epidemiological information on WPVs and cVDPVs is updated weekly on the GPEI website.

WHO does not recommend any restriction on travel and/or trade to Mozambique based on the information available for this current event.