Current study elucidated that higher measles vaccination rates were correlated with lower SARS CoV-2 CFR in OECD countries, instead, a negative but not significant correlation were seen between DTaP vaccination rates and SARS CoV-2 CFR. To the best of our knowledge, this is the first study assessing whether measles and DTaP vaccination rates had a statistical association with COVID-19 CFR in OECD countries. One of the hypotheses about why mortality is lower in SARS CoV-2 infected younger patients is that childhood vaccination programs gradually become more regular in the recent years. Gold et al. demonstrated that highest MMR vaccination rate was in Germany and the lowest rates were observed in Italy, UK and France in 2002, alongside with CFR was significantly high in Italy, UK and France while it was lower in Germany (7). Present study, using measles vaccination data of OECD of 2018, obtained similar results. Measles vaccination rates of Italy, UK and France were under 95% (93%, 92%, 90%, respectively) while in Germany this rate was 97%. Meanwhile, CFR in Italy, UK and France were quite higher (14%, 14%, 18%, respectively) than CFR in Germany (4%) (2). Even though southern hemisphere countries were excluded from study, Madagascar is remarkable with its lower CFR and higher vaccination rates rather than its neighbor countries such as South Africa (8). While vaccination rates reported to be 85%, 91% for MMR and DTaP, respectively in Madagascar; those rates were recorded as 82%, 81%, respectively in South Africa (9, 10). Besides, in terms of CFR based on COVID-19, South Africa had higher mortality rate than Madagascar within the particular period (8). Given the MMR vaccination program implemented in Madagascar just before SARS CoV-2 pandemic and entire society was vaccinated, it is worth to note that this strict vaccination program of this country might be effective on this relatively lower mortality rate comparing with South Africa (11).
Bacillus Calmette-Guerin (BCG) vaccination is also shown to be protective against SARS CoV-2 in a recent study, conducted in 131 countries (12), by determining ten times elevated mortality rate in countries, where BCG is not in routine vaccination program, compared to the countries, where BCG vaccination is applied (40 million vs 4.28 million). BCG vaccine and other live-attenuated vaccines are known to improve general innate immunity not only for the target disease but also for other infections (13). Another explanation for this, might be of these characteristics of live-attenuated vaccines is that those vaccines develop immune memory (14). Together with this information, live attenuated measles virus vaccine is known to induce immune modulation by the suppression of interferon gamma (IFN-γ) production in long-term (15). Considering the increased mortality rates in the elder is attributed to the higher threshold of interferon-mediated immune responses, this suppression of IFN-γ due to measles vaccination might be another effective mechanism on the lower mortality of pediatric population. In accordance with that, measles vaccine was observed to put up significant resistance against SARS CoV-2 in this study. Further investigators indicated that the rubella or measles vaccine, providing cross immune protection to those viral infections, might be explanatory for the vaccinated children being substantially less affected in China (16).
It is known that CFR caused by SARS CoV-2 is not only affected by vaccination program but also factors like social isolation rules implemented by a country, the number of tests performed, health infrastructure. The most typical example of this situation is Sweden, whose CFR is four times of Norway’s CFR, that these two countries have similar vaccination rates. There is a similar case in Iceland, being another Scandinavian country. Iceland is the country with the lowest CFR rate among OECD countries despite their measles vaccination rates being under the OECD average (94.9%) (5). The isolation rules they have implemented and high test rate is being indicated as the main reason of the CFR rate being this low that Iceland is the country with highest test rate per population among OECD countries (17).