Avian Flu Diary: EID Journal Dispatch: Mpox Epidemiology and Vaccine Effectiveness, England, 2023

Credit ACIP/CDC

#18,266

Two weeks ago the WHO Declared Mpox A PHEIC (Public Health Emergency of International Concern) For the 2nd Time, based the the emergence and spread of a more dangerous clade (Ib) in Central Africa.

At the same time, we've seen a modest resurgence in clade IIb cases around the world, despite 2+ years of vaccination and public health awareness campaigns (see May 2024 NYC HAN Advisory: `Substantial' Increases In Mpox Infections Over Past Few Months).

Many of these clade II cases have reportedly occurred in fully vaccinated individuals, although their infections are generally less severe, leading to concerns that the vaccine's protective effects may wear off over time.

Last March, in ECCMID 2024 Study: Mpox (monkeypox) Antibodies Wane Within A Year of Vaccination) we looked at a study by researchers from Erasmus MC in Rotterdam that found:

. . . recipients of the 2-Dose JYNNEOS/ IMVANEX/ IMVAMUNE mpox vaccine who did not receive a childhood smallpox vaccination (discontinued in the 1970s) experienced substantial drops in their immune response after 12 months.

Another presentation, released at roughly the same time from Sweden (see Immune response to MPXV wanes rapidly after intradermal vaccination with MVA-BN (Jynneos)) found an even quicker loss (> 28 days) of detectable neutralizing antibodies after the second vaccination, writing:

Our findings corroborate previous data showing that intradermal MVA-BN vaccination results in neutralizing antibodies only in a proportion of vaccinees, and that a significant decline occurs already during the first months post-vaccination. Immunity after MPXV infection mounts a higher and more robust neutralizing response. In conclusion, the findings merits the study of booster doses.

Today we've a new report, this time from the UK, which finds that nearly half of new community acquired mpox cases in 2023 were in vaccinated individuals. They note:

Nearly half of outbreak case-patients in 2023 were vaccinated, and there were more cases among those who had received 2 doses of MVA-BN vaccine than among those who had received 1 dose.

This result, they suspect, may have more to do with the risk behavior of some who may feel `protected' by two-doses of the vaccine, than the vaccine itself. Add in a possible gradual loss of protection over time, and you have an increased potential for breakthrough infections.

The link and excerpts from the EID Dispatch follow, after which I'll return with a postscript.

Mpox Epidemiology and Vaccine Effectiveness, England, 2023

Hannah Charles , Katie Thorley, Charlie Turner, Kirsty F. Bennet, Nick Andrews, Marta Bertran, Sema Mandal, Gayatri Amirthalingam, Mary E. Ramsay, Hamish Mohammed, and Katy Sinka
Author affiliation: UK Health Security Agency, London, UK

Abstract

Reported mpox cases in England continued at a low but steady frequency during 2023. Of 137 cases reported in 2023, approximately half were acquired overseas and half were in vaccinated persons. Estimated effectiveness of 2-dose vaccine was 80%, and no vaccinated mpox patient was hospitalized.

In England, after the July 2022 peak in the mpox outbreak (1), which affected primarily gay, bisexual, and other men who have sex with men (GBMSM), cases declined and remained low into 2023 (2). We analyzed the epidemiology of postpeak mpox cases in 2023 in England, describing case-patient characteristics including vaccination status and providing an updated estimate of Modified Vaccinia Ankara–Bavarian Nordic (MVA-BN) vaccine effectiveness (VE).

(SNIP)

Conclusions

The low numbers of mpox cases in 2023 were initially interpreted as the final few cases of the 2022 outbreak (1). However, throughout the year, cases continued steadily, split evenly between imported infections and community transmission. The demographic and behavioral characteristics of mpox case-patients in 2023 remained comparable to those in 2022 (Table 1), indicating that mpox continues to circulate predominately within GBMSM sexual networks.

Nearly half of outbreak case-patients in 2023 were vaccinated, and there were more cases among those who had received 2 doses of MVA-BN vaccine than among those who had received 1 dose. Our analysis, based on full-year data from 2023, demonstrates that VE of 1 or 2 doses remains high (82%). The estimated VE for 2 doses compared with 1 dose was marginally lower, but the difference was not statistically significant.

Considering that first doses will have been given farther in the past than second doses and that 2 doses would be expected to confer more protection, that finding is counterintuitive and may reflect differences in risk behavior among those who came forward for a second dose because they may also be at greater risk for exposure to monkeypox virus.

Our observation is consistent with reports from other high-income countries with outbreaks predominantly among GBMSM. In May 2023, the Chicago Department of Public Health (Chicago, IL, USA) noted that most of the cases reported since mid-April were among men who had received 2 doses of MVA-BN vaccine (9), yet a higher number of first doses had been given compared with second doses overall (10).

We found that no vaccinated persons had been hospitalized for mpox in 2023, indicating that the MVA-BN vaccine probably protects against severe disease requiring hospitalization. Our finding is corroborated by a global case series that found illness among vaccinated persons to be less severe (11).

Among the limitations of our analysis, we were unable to examine VE in different population groups, because of unavailability of corresponding disaggregated coverage data. In addition, hospitalization resulting from clinical need was used as a proxy for severity, a pragmatic decision based on the unavailability of data using an objective measure.

Overall, the experience in England during 2023 was of continued low-level community transmission among GBMSM, as well as imported infections, which will probably continue. Given that ≈20 countries continued to report mpox cases in December 2023 and the World Health Organization assessment that the overall global risk for GBMSM remains moderate (12), continued low-level transmission is likely before elimination is reached. Our findings highlight the value of continued active prevention through vaccination and health promotion to persons at higher risk and ongoing surveillance to examine factors that contribute to continued transmission.

Dr. Charles is a principal epidemiologist at the UKHSA and was involved in the UK response to the global 2022 mpox outbreak. Her particular interest is outbreak investigation and real-time surveillance of sexually transmitted infections.

It was never expected that the JYNNEOS vaccine would be 100% effective, or that it would provide life-long immunity. The 82% VE cited by this report is actually pretty impressive, and it aligns roughly with other studies we've seen (see below).

The duration of protection, however, is less clear. While there has been some discussion of the possible need for booster shots - given the finite supply of the vaccine, and the pressing needs in Africa - it is hard to see how both can be accommodated any time soon.