Estimation of anti-orthopoxvirus immunity in Moscow residents and potential risks of spreading Monkeypox virus

Abstract

WHO has declared the outbreak of monkeypox as a public health emergency of international concern. In less than three months, monkeypox was detected in more than 30 000 people and spread to more than 80 countries around the world. It is believed that the immunity formed to smallpox vaccine can protect from monkeypox infection with high efficiency. The widespread use of Vaccinia virus has not been carried out since the 1980s, which raises the question of the level of residual immunity among the population and the identification of groups requiring priority vaccination. We conducted a cross-sectional serological study of remaining immunity among Moscow residents. To do this, a collection of blood serum samples of age group over 30 years old was formed, an in-house ELISA test system was developed, and a virus neutralization protocol was set up. Serum samples were examined for the presence of IgG antibodies against Vaccinia virus (n=2908), as well as for the ability to neutralize plaque formation with a Vaccinia virus MNIIVP-10 strain (n=299). The results indicate the presence of neutralizing antibody titer of 1/20 or more in 33.3 to 53.2% of people older than 45 years. Among people 30-45 years old who probably have not been vaccinated, the proportion with virus neutralizing antibodies ranged from 3.2 to 6.7%. Despite the higher level of antibodies in age group older than 66 years, the proportion of positive samples in this group was slightly lower than in people aged 46-65 years. The results indicate the priority of vaccination in groups younger than 45, and possibly older than 66 years to ensure the protection of the population in case of spread of monkeypox among Moscow residents. The herd immunity level needed to stop the circulation of the virus should be at least 50.25 - 65.28%.

Keywords: immunity; monkeypox virus (MPXV); neutralizing antibody titers (NAT); orthopoxvirus; vaccinia virus (VACV).

Figure 1

Antibody levels according to age. n = 2908. The relative optical density (ODrel) was calculated for each serum sample, dividing the OD-450 of the sample by the average value of OD-450 K+ (weakly positive control) in this experiment. The obtained values were conditionally divided into 5 groups: negative (ODrel <0.5), doubtful (0.501 < ODrel < 1.0), weakly positive (1.001 < ODrel. < 2.0), positive (2.001 < ODrel. < 4.0) and highly positive (ODrel > 4.001). Percentages have statistically significant differences in any comparisons between ages (max p = 0.031). The maximum percentage of samples obtained from people 30-45 years old showed a negative result in ELISA ((ODrel <0.5), most samples from people 46-65 years old showed doubtful result and the largest number of samples from people 66-80 years old showed a weakly positive result.

Figure 2

VNA in different age cohorts. The frequency of samples with VNA = 0 drops sharply in age cohorts older than 45 years. For people younger than 45 years old, the proportion of people with VNA ≥ 1/20 titer ranged from 3.2 to 6.7%, and for people older than 45 years old – from 33.3 to 53.2%.

Figure 3

Dependence of the presence of viral neutralizing activity of blood serum sample on the age group, dilution - 1/20 and more. Y-axis shows the frequency of samples with VNA ≥ 1/20 (%), whiskers show a 95% confidence interval calculated by the Wald method. Two groups compared using chi-square test.

Figure 4

ROC curves for determining the cut off at which it is possible to predict viral neutralizing activity. (A) – All age cohorts, n = 299, AUC = 0.833 (95% CI: 0.787 - 0,880), p<0.001; (B) – With division by age/Blue – 30-45 years old (n=92), AUC = 0.940 (95% CI: 0.879 – 1.000), p=0.001; Red – 46-65 years old (n=118), AUC=0.734 (95% CI: 0.643 – 0.824), p<0.001; Green – 66-80 years old (n=89), AUC=0.759 (95% CI: 0.658 – 0.860), p<0.001.

Figure 5

The level of antibodies to smallpox vaccine depending on the age cohort (A) and the titer of the VNA (B). (A) shows significant differences in IgGs level between samples from 46-65 years old people and 66-80 years old people (Mann Whitney criterion). Whiskers shows maximum and minimum. (B) shows VNA titer ratio in 46-65 and 66-80 years old. p=0.333 (exact Fisher’s test).