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Incremental benefits of male HPV vaccination: accounting for inequality in population uptake.

Smith MA, Canfell K - PLoS ONE (2014)

Bottom Line: Cultural and geographic factors associated with HPV vaccine uptake might also influence sexual partner choice; this might impact post-vaccination outcomes.The rate ratio for post-vaccination incident HPV16 in the lowest compared to the highest coverage subgroup (RR(L)) was calculated to quantify between-group differences in outcomes.The population-level incremental impact of adding males was lower if vaccine uptake was "correlated", however the difference in population-level impact was extremely small (<1%) in the Australia and USA scenarios, even under the conservative and extreme assumption that subgroups according to coverage did not mix at all sexually.

View Article: PubMed Central - PubMed

Affiliation: School of Public Health, The University of Sydney, Sydney, Australia; Prince of Wales Clinical School, UNSW Australia, Sydney, Australia.

ABSTRACT

Background: Vaccines against HPV16/18 are approved for use in females and males but most countries currently have female-only programs. Cultural and geographic factors associated with HPV vaccine uptake might also influence sexual partner choice; this might impact post-vaccination outcomes. Our aims were to examine the population-level impact of adding males to HPV vaccination programs if factors influencing vaccine uptake also influence partner choice, and additionally to quantify how this changes the post-vaccination distribution of disease between subgroups, using incident infections as the outcome measure.

Methods: A dynamic model simulated vaccination of pre-adolescents in two scenarios: 1) vaccine uptake was correlated with factors which also affect sexual partner choice ("correlated"); 2) vaccine uptake was unrelated to these factors ("unrelated"). Coverage and degree of heterogeneity in uptake were informed by observed data from Australia and the USA. Population impact was examined via the effect on incident HPV16 infections. The rate ratio for post-vaccination incident HPV16 in the lowest compared to the highest coverage subgroup (RR(L)) was calculated to quantify between-group differences in outcomes.

Results: The population-level incremental impact of adding males was lower if vaccine uptake was "correlated", however the difference in population-level impact was extremely small (<1%) in the Australia and USA scenarios, even under the conservative and extreme assumption that subgroups according to coverage did not mix at all sexually. At the subgroup level, "correlated" female-only vaccination resulted in RR(L)= 1.9 (Australia) and 1.5 (USA) in females, and RR(L)= 1.5 and 1.3 in males. "Correlated" both-sex vaccination increased RR(L) to 4.2 and 2.1 in females and 3.9 and 2.0 in males in the Australia and USA scenarios respectively.

Conclusions: The population-level incremental impact of male vaccination is unlikely to be substantially impacted by feasible levels of heterogeneity in uptake. However, these findings emphasize the continuing importance of prioritizing high coverage across all groups in HPV vaccination programs in terms of achieving equality of outcomes.

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Impact of heterogeneity in vaccine uptake on population level outcomes.(A) Female-only program (50% overall coverage, extreme inequality). (B) Both sex program (50% overall coverage, extreme inequality). “Correlated” uptake refers to a situation where vaccine uptake within the population is correlated with factors which also affect choice of sexual partners. “Unrelated” uptake refers to a situation where vaccine uptake is unrelated to any of these factors. Vaccination was assumed to commence in 2007.
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pone-0101048-g002: Impact of heterogeneity in vaccine uptake on population level outcomes.(A) Female-only program (50% overall coverage, extreme inequality). (B) Both sex program (50% overall coverage, extreme inequality). “Correlated” uptake refers to a situation where vaccine uptake within the population is correlated with factors which also affect choice of sexual partners. “Unrelated” uptake refers to a situation where vaccine uptake is unrelated to any of these factors. Vaccination was assumed to commence in 2007.

Mentions: In a female-only program with 50% overall coverage, the predicted long-term relative reductions in female and male HPV infections are 56% and 49% for “correlated” uptake under assumptions of ‘extreme inequality’, compared to 62% and 41% for “unrelated” uptake, respectively (Figure 2a). If males were also vaccinated at the same coverage as females, the predicted reductions in female and male infections are 61% and 60% for “correlated” uptake with extreme inequality, compared to 79% and 78% for “unrelated” uptake, respectively (Figure 2b). Similar effects were seen in the “Australia” (higher) and “USA” (lower) coverage scenarios, where the heterogeneity was less pronounced, but in these scenarios the effects were extremely small (<1% difference in population impact) (Table 1). Generally “correlated” uptake resulted in a lower vaccination effectiveness at the population level, except that female-only programs with “correlated” uptake resulted in better outcomes in males (but not in females), than female-only programs with equivalent “unrelated” uptake. The extent to which this occurred varied depending on the degree of heterogeneity in uptake between subgroups, but can be explained by considering the implications in the simple “extreme inequality” scenario. In this example, the assumption that coverage occurred in equal-sized subgroups, one with higher (90%) and one with lower (10%) coverage resulted in a net loss of benefit for females, because the extent of indirect (herd) protection was lower in both of the subgroups than in the scenario with moderate (50%) coverage in females which was uniform across the population. Females in the lower coverage subgroup did not experience a large herd effect because coverage was very low; and within the higher coverage subgroup, there was less room for improvement in outcomes due to herd effects, since coverage, and impact, was already very high for females. In contrast, males who partnered within the subgroup of females with very high uptake experienced strong herd effects. As the benefits to males from a female-only program do not scale linearly with coverage, and appear to accelerate as coverage in females increases [43], [44], the gains in this subgroup of males outweighed the loss for males in the lower coverage subgroup.


Incremental benefits of male HPV vaccination: accounting for inequality in population uptake.

Smith MA, Canfell K - PLoS ONE (2014)

Impact of heterogeneity in vaccine uptake on population level outcomes.(A) Female-only program (50% overall coverage, extreme inequality). (B) Both sex program (50% overall coverage, extreme inequality). “Correlated” uptake refers to a situation where vaccine uptake within the population is correlated with factors which also affect choice of sexual partners. “Unrelated” uptake refers to a situation where vaccine uptake is unrelated to any of these factors. Vaccination was assumed to commence in 2007.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4121069&req=5

pone-0101048-g002: Impact of heterogeneity in vaccine uptake on population level outcomes.(A) Female-only program (50% overall coverage, extreme inequality). (B) Both sex program (50% overall coverage, extreme inequality). “Correlated” uptake refers to a situation where vaccine uptake within the population is correlated with factors which also affect choice of sexual partners. “Unrelated” uptake refers to a situation where vaccine uptake is unrelated to any of these factors. Vaccination was assumed to commence in 2007.
Mentions: In a female-only program with 50% overall coverage, the predicted long-term relative reductions in female and male HPV infections are 56% and 49% for “correlated” uptake under assumptions of ‘extreme inequality’, compared to 62% and 41% for “unrelated” uptake, respectively (Figure 2a). If males were also vaccinated at the same coverage as females, the predicted reductions in female and male infections are 61% and 60% for “correlated” uptake with extreme inequality, compared to 79% and 78% for “unrelated” uptake, respectively (Figure 2b). Similar effects were seen in the “Australia” (higher) and “USA” (lower) coverage scenarios, where the heterogeneity was less pronounced, but in these scenarios the effects were extremely small (<1% difference in population impact) (Table 1). Generally “correlated” uptake resulted in a lower vaccination effectiveness at the population level, except that female-only programs with “correlated” uptake resulted in better outcomes in males (but not in females), than female-only programs with equivalent “unrelated” uptake. The extent to which this occurred varied depending on the degree of heterogeneity in uptake between subgroups, but can be explained by considering the implications in the simple “extreme inequality” scenario. In this example, the assumption that coverage occurred in equal-sized subgroups, one with higher (90%) and one with lower (10%) coverage resulted in a net loss of benefit for females, because the extent of indirect (herd) protection was lower in both of the subgroups than in the scenario with moderate (50%) coverage in females which was uniform across the population. Females in the lower coverage subgroup did not experience a large herd effect because coverage was very low; and within the higher coverage subgroup, there was less room for improvement in outcomes due to herd effects, since coverage, and impact, was already very high for females. In contrast, males who partnered within the subgroup of females with very high uptake experienced strong herd effects. As the benefits to males from a female-only program do not scale linearly with coverage, and appear to accelerate as coverage in females increases [43], [44], the gains in this subgroup of males outweighed the loss for males in the lower coverage subgroup.

Bottom Line: Cultural and geographic factors associated with HPV vaccine uptake might also influence sexual partner choice; this might impact post-vaccination outcomes.The rate ratio for post-vaccination incident HPV16 in the lowest compared to the highest coverage subgroup (RR(L)) was calculated to quantify between-group differences in outcomes.The population-level incremental impact of adding males was lower if vaccine uptake was "correlated", however the difference in population-level impact was extremely small (<1%) in the Australia and USA scenarios, even under the conservative and extreme assumption that subgroups according to coverage did not mix at all sexually.

View Article: PubMed Central - PubMed

Affiliation: School of Public Health, The University of Sydney, Sydney, Australia; Prince of Wales Clinical School, UNSW Australia, Sydney, Australia.

ABSTRACT

Background: Vaccines against HPV16/18 are approved for use in females and males but most countries currently have female-only programs. Cultural and geographic factors associated with HPV vaccine uptake might also influence sexual partner choice; this might impact post-vaccination outcomes. Our aims were to examine the population-level impact of adding males to HPV vaccination programs if factors influencing vaccine uptake also influence partner choice, and additionally to quantify how this changes the post-vaccination distribution of disease between subgroups, using incident infections as the outcome measure.

Methods: A dynamic model simulated vaccination of pre-adolescents in two scenarios: 1) vaccine uptake was correlated with factors which also affect sexual partner choice ("correlated"); 2) vaccine uptake was unrelated to these factors ("unrelated"). Coverage and degree of heterogeneity in uptake were informed by observed data from Australia and the USA. Population impact was examined via the effect on incident HPV16 infections. The rate ratio for post-vaccination incident HPV16 in the lowest compared to the highest coverage subgroup (RR(L)) was calculated to quantify between-group differences in outcomes.

Results: The population-level incremental impact of adding males was lower if vaccine uptake was "correlated", however the difference in population-level impact was extremely small (<1%) in the Australia and USA scenarios, even under the conservative and extreme assumption that subgroups according to coverage did not mix at all sexually. At the subgroup level, "correlated" female-only vaccination resulted in RR(L)= 1.9 (Australia) and 1.5 (USA) in females, and RR(L)= 1.5 and 1.3 in males. "Correlated" both-sex vaccination increased RR(L) to 4.2 and 2.1 in females and 3.9 and 2.0 in males in the Australia and USA scenarios respectively.

Conclusions: The population-level incremental impact of male vaccination is unlikely to be substantially impacted by feasible levels of heterogeneity in uptake. However, these findings emphasize the continuing importance of prioritizing high coverage across all groups in HPV vaccination programs in terms of achieving equality of outcomes.

Show MeSH
Related in: MedlinePlus