Effectiveness of oral polio vaccination against paralytic poliomyelitis: a matched case-control study in Somalia

Long-Term Immunogenicity of Inactivated and Oral Polio Vaccines: An Italian Retrospective Cohort Study

Oral and inactivated poliovirus (PV) vaccines have contributed toward the global eradication of wild PV2 and PV3, as well as the elimination of PV1 in most countries. While the long-term (>5−10 years) persistence of protective antibodies in ≥80% of the population vaccinated with ≥3−4 doses of oral poliovirus vaccine (OPV) has been demonstrated, the duration of immunity in people vaccinated with the inactivated poliovirus vaccine (IPV) is still unclear. This study evaluated the seroprevalence of anti-PV neutralizing antibodies and the long-term immunogenicity conferred by OPV and IPV in a sample of medical students from the University of Bari (April 2014−October 2020). The levels of neutralizing PV1, PV2, and PV3 antibodies in blood samples taken during the assessments were evaluated. Neutralizing antibodies against PV1, PV2, and PV3 were present in >90% of the study participants, with rates of >99%, >98%, and ~92−99%, respectively. IPV resulted in a higher immunological response than OPV against PV3. Protective antibodies against all three viruses persisted for at least 18 years after administration of the last vaccine dose. Until PV1 is completely eradicated, maximum vigilance from public health institutions must be maintained.

Estimation of oral poliovirus vaccine effectiveness in Afghanistan, 2010-2020

BACKGROUND

Afghanistan is one of two countries with endemic wild poliovirus type 1 (WPV1). The oral poliovirus vaccine (OPV) is the predominant vaccine used for polio eradication. Although OPV has been administered in routine childhood immunization and during frequent supplementary immunization activities, WPV1 continues to circulate in Afghanistan and case incidence has been increasing since 2017. We estimated the effectiveness of OPV in Afghanistan during 2010-2020.

METHODS

We conducted a matched case-control analysis using acute flaccid paralysis (AFP) surveillance data from 29,370 children < 15 years with AFP onset between January 1, 2010 and December 31, 2020. We matched children with confirmed WPV1 (cases) with children with non-polio AFP (controls) by age at onset of paralysis (+/- 3 months), date of onset of paralysis (+/- 3 months), and province of residence, and compared their reported OPV vaccination history to estimate the effectiveness of OPV in preventing paralysis by WPV1 using conditional logistic regression. To account for changes in OPV formulations provided over the analysis period, we stratified the analysis based on dates of the global switch from trivalent OPV (tOPV) to bivalent OPV (bOPV) in April 2016.

RESULTS

Between January 1, 2010 and December 31, 2020, there were 329 WPV1 cases in Afghanistan. The per-dose estimated effectiveness of OPV against WPV1 was 19% (95% CI: 15%-22%) and of ≥ 7 doses was 94% (95% CI: 90%-97%). Before the global switch from tOPV to bOPV, the per-dose estimated effectiveness of OPV was 14% (95% CI: 11%-18%) and of ≥ 7 doses was 92% (95% CI: 85%-96%). After the switch, the per-dose estimated effectiveness of OPV against WPV1 was 32% (24%-39%) and of ≥ 7 doses was 96% (95% CI: 90%-99%).

DISCUSSION

OPV is highly effective in preventing paralysis by WPV1; these results indicate that continued WPV1 transmission in Afghanistan is due to failure to vaccinate, not failure of the vaccine. Although difficult to implement in parts of country, improving the administration of OPV in routine immunization and supplementary immunization activities will be critical for achieving polio eradication in Afghanistan.

Using Nonpolio Enterovirus Detection to Assess the Integrity of Stool Specimens Collected From Acute Flaccid Paralysis Cases in Somalia During 2014-2017

Background

Despite insecurity challenges in Somalia, key indicators for acute flaccid paralysis (AFP) surveillance have met recommended targets. However, recent outbreaks of vaccine-derived polioviruses have raised concerns about possible gaps. We analyzed nonpolio enterovirus (NPEV) and Sabin poliovirus isolation rates to investigate whether comparing these rates can inform about the integrity of stool specimens from inaccessible areas and the likelihood of detecting circulating polioviruses.

Methods

Using logistic regression, we analyzed case-based AFP surveillance data for 1348 cases with onset during 2014-2017. We assessed the adjusted impacts of variables including age, accessibility, and Sabin-like virus isolation on NPEV detection.

Results

NPEVs were more likely to be isolated from AFP case patients reported from inaccessible areas than accessible areas (23% vs 15%; P = .01). In a multivariable model, inaccessibility and detection of Sabin-like virus were positively associated with NPEV detection (adjusted odds ratio [AOR], 1.75; 95% confidence interval [CI], 1.14-2.65; and AOR, 1.79; 95% CI, 1.07-2.90; respectively), while being aged ≥5 years was negatively associated (AOR, 0.42; 95% CI, 0.20-0.85).

Conclusions

Rates of NPEV and Sabin poliovirus detection in inaccessible areas suggest that the integrity of fecal specimens tested for AFP surveillance in Somalia can generate useful AFP data, but uncertainties remain about surveillance system quality.

The Use of Test-negative Controls to Monitor Vaccine Effectiveness: A Systematic Review of Methodology

BACKGROUND

The test-negative design is an increasingly popular approach for estimating vaccine effectiveness (VE) due to its efficiency. This review aims to examine published test-negative design studies of VE and to explore similarities and differences in methodological choices for different diseases and vaccines.

METHODS

We conducted a systematic search on PubMed, Web of Science, and Medline, for studies reporting the effectiveness of any vaccines using a test-negative design. We screened titles and abstracts and reviewed full texts to identify relevant articles. We created a standardized form for each included article to extract information on the pathogen of interest, vaccine(s) being evaluated, study setting, clinical case definition, choices of cases and controls, and statistical approaches used to estimate VE.

RESULTS

We identified a total of 348 articles, including studies on VE against influenza virus (n = 253), rotavirus (n = 48), pneumococcus (n = 24), and nine other pathogens. Clinical case definitions used to enroll patients were similar by pathogens of interest but the sets of symptoms that defined them varied substantially. Controls could be those testing negative for the pathogen of interest, those testing positive for nonvaccine type of the pathogen of interest, or a subset of those testing positive for alternative pathogens. Most studies controlled for age, calendar time, and comorbidities.

CONCLUSIONS

Our review highlights similarities and differences in the application of the test-negative design that deserve further examination. If vaccination reduces disease severity in breakthrough infections, particular care must be taken in interpreting vaccine effectiveness estimates from test-negative design studies.

Strengthening Acute Flaccid Paralysis Surveillance Through the Village Polio Volunteers Program in Somalia

Background

Surveillance for cases of acute flaccid paralysis (AFP) is a key strategy adopted for the eradication of polio. Detection of poliovirus circulation is often predicated on the ability to identify AFP cases and test their stool specimens for poliovirus infection in a timely manner. The Village Polio Volunteers (VPV) program was established in 2013 in a bid to strengthen polio eradication activities in Somalia, including AFP surveillance, given the country's vulnerability to polio outbreaks.

Methods

To assess the impact of the VPV program on AFP surveillance, we determined case counts, case-reporting sources, and nonpolio AFP rates in the years before and after program introduction (ie, 2011-2016). We also compared the stool specimen adequacy rates and timeliness of cases reported by VPVs to those reported by other sources.

Results

In the years after program introduction, VPVs accounted for a high proportion of AFP cases reported in Somalia. AFP case counts rose from 148 cases in 2012, the year before program introduction, to 279 cases in 2015, when VPVs accounted for 40% of reported cases. Further, from 2012 to 2015, the nonpolio AFP rate improved from 2.8 to 4.8 cases per 100000 persons aged <15 years. Stool specimen adequacy rates have been consistently high, and AFP cases have been detected in a timelier manner since the program was introduced.

Conclusions

Given the impact of the VPV program on improving AFP surveillance indicators in Somalia, similar community-based programs could play a crucial role in enhancing surveillance activities in countries with limited healthcare infrastructure.

Contribution of Contact Sampling in Increasing Sensitivity of Poliovirus Detection During A Polio Outbreak—Somalia, 2013

Background.  In May 2013, a wild poliovirus type 1 (WPV1) outbreak reported in Somalia provided an opportunity to examine the contribution of testing contacts to WPV detection.

Methods.  We reviewed acute flaccid paralysis (AFP) case-patients and linked contacts reported in the Somalia Surveillance Database from May 9 to December 31, 2013. We restricted our analysis to AFP case-patients that had ≥3 contacts and calculated the contribution of each contact to case detection.

Results.  Among 546 AFP cases identified, 328 AFP cases had ≥3 contacts. Among the 328 AFP cases with ≥3 contacts, 93 WPV1 cases were detected: 58 cases (62%; 95% confidence interval [CI], 52%–72%) were detected through testing stool specimens from AFP case-patients; and 35 cases (38%; 95% CI, 28%–48%) were detected through testing stool specimens from contacts, including 19 cases (20%; 95% CI, 14%–30%) from the first contact, 11 cases (12%; 95% CI, 7%–20%) from the second contact, and 5 cases (5%; 95% CI, 2%–12%) from the third contact. Among the 103 AFP cases with ≥4 contacts, 3 (6%; 95% CI, 2%–16%) of 52 WPV1 cases were detected by testing the fourth contact. No additional WPV1 cases were detected by testing &gt;4 contacts.

Conclusions.  Stool specimens from 3 to 4 contacts of persons with AFP during polio outbreaks are needed to maximize detection of WPV cases.