People with blood type O more protected against COVID-19 - studies

Blood Test

If a person with blood type A transmits the virus to a person with type O blood, the type O person will have antibodies that can fight the virus.

People with blood type O have less of a chance of getting COVID-19, according to a number of studies published all over the world during the pandemic. But people with type A might be more vulnerable. 
After the pandemic broke out, one of the questions that scientists and doctors became puzzled by was why are some people more susceptible to contracting and even succumbing to the disease caused by coronavirus than others?
Several studies have researched the link that genetics plays in susceptibility to contracting the virus, and some more specifically have linked the role played by blood type. The studies all came to a similar conclusion: People with blood type O have less of a chance of getting COVID-19. 
It should be noted that many studies conducted by reputable organizations are not fully peer-reviewed due to the urgency of the data, and there are some who warn that the data cannot be accepted as fact. 
Two studies came out of Wuhan, the epicenter of the COVID-19 outbreak, one that researched the link between blood type and COVID-19 susceptibility, and the other that investigated the link between blood type and the risk of contracting pneumonia caused by SARS‐CoV‐2 – the novel coronavirus. Another study came out of Italy and Spain, which looked into the link between genetics and SARS-CoV-2 respiratory failure. 
But apparently the most popular, as well as most recent study is the ongoing research published by the personal genetics company 23andMe regarding evidence that blood type plays a role in COVID-19. 23andMe is a company that sends out personal genetics testing kits to individuals who are interested in finding out their genetic history and or their predisposition to certain genetically transmitted diseases such as certain types of cancer. 
According to the company's blog, they did a study that contained more than 750,000 participants. Their preliminary data suggests that O blood type appears to be protective against the virus, and people with that type are between 9-18% less likely than other individuals to have tested positive for the COVID-19 virus. 
But how does blood type actually play a role in infection? 
WHILE THE answer is not exactly known, Andre Franke, the lead scientist from the study done in Spain and Italy, suggested an answer to The New York Times. He noted that the gene region that codes for blood type is associated with elevated levels of key immune molecules.
Their study included extracting DNA samples from 1,980 COVID-19 patients who were hospitalized for respiratory failure. They scanned the samples using a rapid technique called genotyping, and looked at nearly nine million genetic letters. The researchers then carried out the same experiment on 2,205 blood donors with no evidence of COVID-19. They found that the ill patients shared a number of similar genetic variants compared to those who were not ill.  
The scientists were looking for spots in the genome called loci, which they ultimately found. The gene that determines blood type is in one of those spots. Their analysis showed a higher risk for A-positive individuals and a protective effect for blood group O. 
The researchers in Wuhan came to a similar conclusion. After eliminating other confounding risk factors such as age, gender and comorbidity, the team investigated and confirmed a connection between people with ABO blood groups and a risk of SARS-CoV-2 pneumonia in patients from three hospitals in Wuhan. In short, their findings also found that people with blood type A were at higher risk of hospitalization while O patients had the lowest risk. 
Other past research done in 2005 during the 2002 SARS epidemic supports the O blood type protection theory. Research at a Hong Kong hospital analyzed an infected patient who exposed 45 healthcare workers. Of 18 people with type blood O, 8 became infected (44%), compared to the other 27 people having other blood types, of whom 23 contracted the virus (85% – almost twice as much).
BLOOD TYPE is determined by specific sugar molecules that are added to proteins or lipids on our blood cells and other cell types, according to Chemical Science and Engineering News. Those that are in the A blood type group carry A-sugar-antigens; the same goes for people with blood type B having B-antigens. But those with O blood type have neither. 
This means that the immune systems of people with type A blood develop antibodies for B antigens, people with type B blood have antibodies for A antigens, and people with type O blood have antibodies for both. Blood type influences blood clotting, and a growing body of evidence suggests that COVID-19 pathology often involves overactive blood clotting. People with type O blood have lower levels of proteins that promote blood clotting.
SARS-CoV-2 can replicate in cells that express blood type antigens, Jacques Le Pendu, a glycobiologist at the University of Nantes told Chemical and Engineering News. This means that when an infected person coughs or sneezes, there's a possibility that they release viral particles coated with their blood type antigens.
Explained more in depth, a person in the blood type O group will have antibodies against virus transmission from someone in the blood type A group, which can fight the virus. However, a person in the blood type A group won't have those same antibodies. 

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