Using poliomyelitis as a precedent, this investigation shows that current immunization recording in Germany is not fit for the next pandemic. While 23% of volunteers had an incomplete vaccination status according to the recommended schedule besides fully available individual vaccination records, more than 40% of participants were classified with uncertain vaccination status, mostly due to the lack of specific details on previously administered doses.
For vaccines less established than those against PM, or less often part of a combination vaccine, rates must be assumed worse [9, 24]. The rates found in our study appear even direr when seen against a background of successful global mass PM vaccination campaigning, and considering that adult volunteers in the VACCELERATE registry may likely more interested in the topic [9, 21].
Pandemic preparedness comprises knowledge of vaccination gaps at a population-level [2]. The (re-)emergence of highly contagious and vaccine preventable pathogens demands for broad, rapid and specific knowledge of previously administered vaccine doses to target those with incomplete vaccination schedules [2, 3]. Sufficient vaccination coverage is required to induce protective population immunity and prevent outbreaks [28]. In the absence of a centralized vaccination registry, growing vaccine hesitancy and with an increased level of migration, data on individual previous immunization can only be derived from personal vaccination certificates, medical records or self-reports [5, 18, 20].
The last evaluation of the PM vaccination status in German adults was accomplished over ten years ago as part of the first wave of German Health Interview and Examination Survey for Adults (DEGS1) [16]. Data collected from vaccination cards and self-reports thereby revealed a lifetime prevalence for at least one PM vaccination of 85.6% [16]. In the present study, we found a slightly higher lifetime vaccination prevalence with 93.1% among registered volunteers. Recent public warnings after PV detection in non-endemic regions may have motivated people to check their PM immunization status [29]. However, our results showed no significant increase in vaccination rates in 2022 compared to the four preceding years, while most doses were recorded in 2020 at the beginning of the COVID-19 pandemic.
A 2002 PM seroprevalence study on PM in Germany including sera from 2.564 adults indicated high levels of population immunity showing neutralizing antibodies against PV type 1 in 96.8% of subjects, as well as 96.8% against type 2 and 89.6% type 3 respectively [30]. This study also evaluated regional differences in immunization status. Higher rates of sufficient PM immunization were assessed in sera from individuals from Western German states [30]. In our study, no significant differences in reported PM vaccination status were found in volunteers from Western versus Eastern states. These geographic analyses warrant caution as volunteers may have moved between regions.
Similar to our findings, there was an association between demographics and vaccination status in DEGS1, showing higher vaccination rates in women compared to men and in younger compared to older participants [16]. Highest sex related difference in vaccination rates was found in both, DEGS1 and our study, in the 50–59 years age group. Higher vaccination rates in women were also reported for other vaccine preventable diseases and geographic locations for example influenza in the US and COVID-19 in Canada [31, 32]. As these are not childhood vaccinations like PM, sex-specific differences in health care adherence and decision-making that might have played a role there are less plausible causes regarding PM vaccination rates [31, 33].
In DEGS1, women were able to present their vaccination certificate more often [16]. Highest rates of complete vaccination records were found in women between 18 and 29 years and lowest rates in men between 60 and 69 years [16]. Our study showed similar findings. Year of birth and oldest available certificate entry was higher in female and younger participants. Not surprisingly, we found that the closer the oldest available vaccination certificate was to birth year, the more likely immunization records were complete. We assume incomplete vaccination records and increasing recall bias over time are main causes for reporting incomplete vaccination status. Each vaccine obtained in Germany is documented in a personal paper-based vaccine certificate [16]. Seasonal vaccines, e.g. influenza, are not necessarily documented here. If space for documentation is used up, individuals may obtain a new certificate. As there is no integral harmonized vaccination recording in Germany, data on previous vaccines can be lacking if a particular certificate is lost [5].
Our study has several limitations. It is questionable whether our sample is representative as participants of the VACCELERATE volunteer registry may be more interested in vaccination, suggesting that the dark figure of incomplete vaccination might be even higher. Nevertheless, a projection for Germany based upon our data would still result in a considerable number of at least 19.719.326 (23.4%) individuals with incomplete vaccination status potentially at risk when exposed to PV [27]. Hypothetically, seropositivity might be higher taking the high number of volunteers with any vaccination against PM into account. The calculated herd immunity threshold of 80 to 86% would not be reached according to our study data [28]. It is important to note that this study is limited by the fact that serological analyses were not included. The study was not meant to assess seroprevalence of protection against PV but collect available data on prior administered vaccine doses to identify potential vaccination gaps at a population-level and inform participants in case of incomplete vaccination schedule.
Further limitations consist of recall and/or reporting bias as vaccines were often administered many years ago, and data transfer from a paper-based vaccination certificate into an eCRF can be erroneous. Future PM vaccination status evaluations may use technical assistance for automated reading of vaccination certificates, such as the Vacuna® app employing image recognition [34].
Ideally, vaccinations were recorded electronically in a centralized registry to allow for permanent and online-availability, and for international harmonization as individually introduced for COVID-19 vaccination during the pandemic.[4] In daily routine, missing vaccination records cause uncertainty among treating physicians as well as time-consuming investigations by medical staff [25]. The involvement of electronic tools would allow verification of self-reported data [34].
In conclusion, PM vaccination status was difficult to assess due to absent or incomplete records. The current paper-based vaccination recording system in Germany fails to inform the individual citizen and public health authorities. Further investigations including serological analyses adding to the data collected in this study are needed to elucidate the population PM immunization status, especially considering vulnerable patient groups with uncertain or incomplete vaccination history.