COVID-19 Fact Check

The coronavirus disease 2019 (COVID-19) epidemic is certainly cause for concern. Proper communication and optimal decision-making is an ongoing challenge, as data evolve. The challenge is complex, especially caused by exaggerated information which can lead to inappropriate actions. It is important to differentiate promptly the true epidemic from an epidemic of false claims and potentially harmful actions.

This is why CEBaP searches for COVID-19 literature, which is relevant for the Red Cross, on a daily basis and assesses new studies on their reliability. Here we provide an overview of our analyses of the identified articles.

Do air temperature and humidity influence the COVID-19 disease outbreak? (Update April 23, 2020)

A systematic review which included 17 observational studies reported a negative association between the spread of COVID-19 and temperature and humidity in the majority of included studies. That is, cold and dry conditions were associated with an increased spread of COVID-19, whereas warm and wet climates were associated with a reduced spread of COVID-19.

Seven of the 17 studies in this systematic review were conducted in China, while the other 10 studies included data from several different countries affected by the novel coronavirus. Caution is warranted when interpreting these results:

  1. The systematic review and some of its included studies have not (yet) been peer-reviewed.
  2. The systematic review states that sixteen articles were unanimous in stating that cool and dry conditions were potentiating factors for the spread of COVID-19, but two of those studies did not report this straightforward association, both for the temperature and the humidity variable.
  3. The reported results are of low-certainty evidence due to the study design (observational studies) and the considerable risk of bias (mostly due to limitations in sample selection criteria and confounding factors) of included studies.
  4. Most of the studies did not account for possible confounding factors that could influence the spread of COVID-19 such as migration patterns, containment measures, general health policies, population density, herd immunity, transportation, and cultural aspects.


Based on this review, we cannot be certain about the impact of air temperature and humidity on COVID-19 incidence and viral transmission. The association only explains a portion of the variation, while there are many confounding variables which may have a larger impact. Any decline of virus transmission during the summer due to temperature and humidity is expected to be modest, and not enough to stop transmission on its own. In the meantime, everyone should adhere to the restrictions imposed by the government to limit the spread of COVID-19.

The Centre for Evidence-Based Medicine (CEBM) of Oxford University performed a similar fact-check on this topic, find out their verdict here.

Do infants and young children have more severe COVID-19 symptoms? (March 24, 2020)

A study on 2.143 pediatric Chinese patients with 2019 Coronavirus Disease reported that children have generally less severe COVID-19 symptoms compared to adults, and infants and young children (below 5 years of age) are more susceptible to severe COVID-19 symptoms compared to older children.

After a critical appraisal of the study methodology underlying these statements, we concluded that the association between children’s age and the severity of COVID-19 symptoms is very uncertain and information that infants and young children are more prone to severe COVID-19 symptoms, is misleading.

There are 2 main reasons why we concluded that this study is misleading:

  1. An objective laboratory-confirmed diagnosis of COVID-19 was only reported in about 30% of the children. The remaining 70% COVID-19 cases were defined as ‘suspected’ and no laboratory confirmation was available.
  2. Also the categories used to define the severity of COVID-19 (asymptomatic, mild, moderate, severe and critical) and children’s age (<1, 1-5, 6-10, 11-15, >15 years) are arbitrarily chosen and prone to subjectivity.
    This is an observational study (cross-sectional study design) where potential confounding factors, such as underlying (lung) diseases, co-infections with other viruses or bacteria (e.g. RSV) or other clinical characteristics, might influence the association between age and COVID-19 in children. The study authors did not account for any of these confounding factors in their statistical analysis. Therefore, we cannot exclude that the observation of young children being more susceptible to COVID-19 is determined by one or more of these confounding factors.


The results of this study are misleading. No need to panic for parents of infants or young children.

Let it be clear however that, no matter how young and fit you are, anyone can be infected with COVID-19. Nevertheless, it is also clear that mainly the elderly and people with underlying diseases are the most affected. 

Are people with blood type A at a significantly higher risk of COVID-19? (Update April 23, 2020)

On social media, rumour has it that people with blood type A are more susceptible to COVID-19 than people with blood type O.

The rumour is based on the conclusions of a study that was pre-published on March 11th 2020 (Zhao et al. 2020), followed by a similar study pre-published on April 15th (Zeng et al. 2020). Both studies found a higher percentage of blood type A in a cohort of COVID-19 patients than in the general population. Moreover, in the past, blood types have been associated with the occurrence of a number of diseases. However, based on the data of the two studies at hand, we cannot conclude that people with a certain blood type are more likely to become infected with SARS-CoV-2 and to develop COVID-19 than others.

First of all, both manuscripts have not yet been peer-reviewed.

Second, these observational studies compare a population of patients with a control group representing the general population, but provide no demographic details (age and sex distribution) on their respective control group. In addition, the study investigators did not adjust their analyses for age and sex (or any other potential confounders). Therefore, confounding of the supposed association between SARS-CoV-2 and ABO blood type by demographic differences between cases and controls cannot be ruled out.

Third, generalizability of the results is questionable. Both studies were conducted in China. Zhao et al. included data on ABO blood type and COVID-19 from three hospitals in two agglomerations in China (Wuhan and Shenzen), and they showed substantial heterogeneity in ABO blood type distributions among people with and without the disease in both regions. In the study by Zeng et al., patients from three hospitals (in Beijing, Xi’an and another hospital in Wuhan) were included.

Forth, statistical significance does not always imply clinical relevance. Although results for the association between blood type and hospitalization with SARS-CoV-2 infection were significant, the percentages are not that different after all. Both studies made use of chi squared tests for homogeneity. When the sample size is large (say >500, which is the case here), almost any small difference will appear statistically significant with a chi squared test. Odds ratios were calculated as well, but as pointed out above, these were not adjusted for covariates, and furthermore, they were not very different from the “no effect” value of 1 either.

Finally, infection with SARS-CoV-2 and development of serious COVID-19 outcomes are two different things. Zeng et al. failed to find any association between blood type and clinical outcomes, such as acute respiratory distress syndrome, acute kidney injury, and death. They rightly concluded that “people and health care workers should not overestimate the genetic susceptibility by placing a certain blood group at risk for poor prognosis.”


We conclude that there is no convincing evidence on the association between ABO blood type and SARS-CoV-2 susceptibility, let alone development of serious COVID-19 disease. Everybody, regardless of blood group, should adhere to the restrictions imposed by the government to counter the spread of SARS-CoV-2.


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