At the beginning of the COVID-19 pandemic caused by SARS-CoV-2, the groups with the highest risk to contract the disease were established. Children were quickly quarantined since society understands that they are the future and the most precious thing to protect. However, a recent study published in the prestigious medical journal JAMA (Journal of the American Medical Association) sheds some light on what health professionals were empirically testing. Those under 17 years of age were a group especially resistant to contracting the disease. If this study is combined with another one provided by the Children's Hospital "Hospital Sant Joan de Déu" in Barcelona (Spain), very encouraging results can be observed, even if they still do not have a solid molecular explanation.
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The study, presented in the letter format, "Nasal Gene Expression of Angiotensin-Converting Enzyme 2 in Children and Adults" (translated as "expression of angiotensin-converting enzyme 2 (ACE2) in children and adults") highlights differences in expression of ACE2, the key protein for virus entry into cells. ACE2 is the same one that the SARS coronavirus used to infect human when an outbreak began in Asia a few years ago, making it one of the first to be studied for SARS coronavirus. This protein, which appears in both the lungs and the heart, is also found in the respiratory tract, the main region of entry for infection with SARS group viruses.
The study has used samples taken in recent years at a well-known New York hospital and has 305 individuals between the ages of 4 and 60, with representatives in all groups of both sexes. By means of RNA sequencing, it was verified which proteins were being synthesized in samples of the mucosa of the nasal epithelium. The results regarding ACE2 are very clear, the age group of less than 10 years were those with the least amount of the protein. With age, the presence of the protein increases linearly until it stabilizes around the age of 25, which by then expresses 50% more than in the younger group.
Collaterally, the study has shed light on another of the unknowns of the coronavirus and its entry mediated by ACE2. The samples were collected before knowing anything about the coronavirus and were intended for an asthma study. Thanks to this, it has been observed that ACE2 is found in the same measure in people who suffer from this respiratory affection, so in this aspect it could be ruled out that asthmatics were more likely to contract the disease than the rest of the population. Similarly, when taking a sample that included both men and women, it was found that protein levels were the same for both sexes.
While it is true that the study does not use fluid samples from the lungs, where the entry of the SARS-CoV-2 virus is believed to occur. However, this new study may change what was believed about the entry of the virus into human cells, and therefore the beginning of the infection.
On the other hand, the study carried out at the “Hospital Sant Joan de Déu” in the framework of the project “Kids Corona” showed that despite the prevalence of the disease is similar to adults, the symptoms are much milder. This study is in the same line with others published on child resistance and the low transmission rate they have, such as the one published in the BMJ "Children are not COVID-19 super spreaders: time to go back to school".
The Spanish one was carried out with 724 children in homes where at least one of the parents was positive for COVID-19. In these households 18.9% of the adults who lived together tested positive and 17.5% of the children. However, 99% of the children had mild symptoms or were asymptomatic.
Regarding the presence of virus RNA in the nasopharyngeal tract, 33.8% of adults tested positive one month after infection, but only 11.9% of children still had a sufficient viral load to test positive in a PCR test. Not only is the number of positives lower, but the study also shows that the viral load is also lower than in adults.
The results of this study seem to show some hope in the spread of the disease in the youngest members of the household. But as its authors say, the observation time, which in this case has been two months, must still be increased, and the reasons why the children have this greater resistance must be sought. The research team is looking for markers in the children's microbiota that shed some light on why they are more resistant. Finally, the same group has also carried out a study on pregnant women. Although it is in the preliminary phase, they have advanced that it seems that the less the degree of development of the fetus, the greater the immunity of the mother to COVID-19. Being the pregnant women who have been infected by SARS-CoV-2 during the third trimester those who have presented a more serious symptomatology.
Bunyavanich S, Do A, Vicencio A. Nasal Gene Expression of Angiotensin-Converting Enzyme 2 in Children and Adults. JAMA. Published online May 20, 2020. doi:10.1001/jama.2020.8707
https://www.sjdhospitalbarcelona.org/en/children-have-similar-prevalence-covid-19-antibodies-adults-more-99-have-mild-symptoms. Published online Jun, 09, 2020.