[Date of latest publication cited: November 13, 2020]
Probably less than 1,000 SARS-CoV-2 viruses entering the body within minutes can start infection, called the “infectious dose.” If many more than that enter, then that might cause greater numbers of the viruses to multiply in the body, which might cause more severe disease in some people. If many less than that enter, the immune system can probably fight them off (Karimzadeh et al.; Mandavilli “It’s not whether”). This can occur over 15 consecutive minutes, or a cumulative 15 minutes in one day (Centers for Disease Control and Prevention “Coronavirus Disease 2019 (COVID-19) Appendices”; Neel, Campbell; Pringle et al.).
Spike proteins on the outside of the virus attach to a protein on the outside of some kinds of human cells, called angio-tensin converting enzyme 2 (ACE2). ACE2 normally regulates other chemicals on the cells, and maintains blood pressure. The virus also hijacks other chemicals on the outside of human cells to enter the cells, especially transmembrane protease serine 2 (TMPRSS2), which normally helps the cell take proteins apart so the cell can use the amino acid parts to make other proteins. The spike protein also binds to neuropilin-1 (NRP1) on the outside of cells, in order to enter the cells, which are abundant inside the nose (Cantuti-Castrelvetri et al.; Daly et al.; Kielian). So, SARS-CoV-2 infects cells which have ACE2, TMPRSS2, and NRP1, and causes COVID-19 disease. The nose has a lot of ACE2, with the eyes, lungs, intestines, and other organs having lesser amounts (Ortiz et al.; Yang et al.). So, the infection usually starts in the nose and eyes, spreads through the respiratory system, and then to many organs (Chen M, Shen et al.; Fischetti et al.; Johns Hopkins Medicine; Matheson, Lehner; Nature; Weirsinga et al.; Zhou et al.). A study of genes associated with cells and organs with molecules susceptible to SARS-CoV-2 found those in the lungs, heart, intestines, nerves, kidney, placenta, testes, and prostate (Singh et al. “A Single Cell RNA Expression Map. Cornell”; Singh et al. “A Single Cell RNA Expression Map. Cell Reports”; Wiegand).
SARS-CoV-2 multiplies rapidly in the epithelial cells in the internal surfaces of the respiratory system. These are shown in eye-catching photos of hundreds of the virions (whole infectious viruses) on the cilia (hair-like cells), taken by researchers (Ehre; University of North Carolina).
[Date of latest publication cited: October 22, 2020]
In some COVID-19 patients, SARS-CoV-2 might have entered the eyes of those who had not been wearing face shields (Chen X, Yu H, et al.; Lu CW, Liu, et al.). Some patients had prodromal ocular symptoms (eye irritation, conjunctivitis) before onset of disease in other body parts (Hong et al.). RT-PCR found SARS-CoV-2 in the tears and conjunctival secretions of some patients with dry eyes and conjunctivitis (Aiello et al.; Chen L, Liu M et al.; Chen MJ et al.; Chen X, Yu H, et al.; Wu P et al.; Xia et al.; Zhang X et al.), and some patients without eye symptoms (Xie et al.). SARS-CoV-2 replicates competently in conjunctiva (Chen X, Yu H, et al.;Hui et al.) The flow of tears to the nasolacrimal system, and the innate immune system, can prevent most pathogens from entering the eyes. But if SARS-CoV-2 gets past that, and infect some eye cells via their ACE2 receptors (Zhou et al.), then viruses could flush with tears into the nasopharyngeal space in the nose and throat, and then to the respiratory system (Chen X, Yu H, et al.; Hong N, Yu, et al.; Napoli et al.). These events can occur in both children and adults (Beery). SARS-CoV-2 was in tears of 24% of moderate to severe patients (Arora et al.), indicating they potentially could transmit to other people. For these reasons, COVID-19 might be transmitted from an infected person’s eye to others.
Wearing glasses is statistically associated with not getting COVID-19, indicating that perhaps glasses prevent SARS-CoV-2 from entering eyes (Maragakis; Parker-Pope “Does Wearing Glasses Protect”; Zeng et al.). So, people in general should avoid touching near their eyes, and people should wear face shields, safety glasses, or goggles (Chen MJ, Chang, et al.; Lu CW, Liu,et al.; Napoli et al.; Wu P et al.).
[Date of latest publication cited: October 23, 2020]
COVID-19 is probably not sexually transmitted, but might be (Entezami et al.). SARS-CoV-2 RNA was not found in vaginas of infected women, perhaps because vaginal tissue lacks angiotensin-converting enzyme 2 (ACE2), the cell receptor for SARS-CoV-2 (Cui et al.; (Entezami et al.). There is more ACE2 in testes than ovaries, so testes might serve as reservoirs for SARS-CoV-2, which might explain why more men were severely infected than women (Entezami et al.;Shastri et al.). Some studies found SARS-CoV-2 RNA in semen (Li D, Jin, et al.; Shapiro), but others did not find it in semen (Pan F, Xiao, et al.) and expressed prostatic secretions (EPS) of infected men (Entezami et al.; Pan F, Xiao et al.; Quan et al.). COVID-19 infection has impaired sperm production in some men (Li H, Xiao et al.). Scientists do not yet know clearly if SARS-CoV-2 could cause problems for natural reproduction or assisted reproduction technologies (ART) (Entezami et al.).
Even if COVID-19 is probably not transmitted by sexual intercourse, sexual interactions could transmit it through other routes described above (Cipriano et al.). The least risky would be abstinence, higher risk is partners living together, or regular partners not living together. The next higher risk is partners not living together using face masks, and highest risk is having several partners not using masks (Marcus; Solé; Turban, Keuroghlian, Mayer; Tam).
Pregnancy and Newborns
[Date of latest publication cited: October 26, 2020]
When COVID-19 infected pregnant women gave birth, most infants were not infected. A few were infected briefly, however, indicating possible intrauterine vertical transmission from the mother through the placenta to the fetus (Chen H, Guo, et al.; Chen Y, Peng, et al.; Kelvin and Halperin; Mullins et al.; Qiao; Shalby; Van Beusekom “COVID19”; Yu N, Li et al.; Zeng et al.; Zhang Z, Yu, et al.). Most of those infected during pregnancy soon recovered (Rodriguez; Vivanti et al.). One mother had SARS-CoV-2 in her breast milk for 4 days, and her newborn was briefly infected. It was unclear how the newborn was infected (Groß et al.). After a baby is born, the mother and her family could transmit viruses to the baby by the same routes as any other person.
Some cells and tissues between pregnant women and their fetuses (the maternal-fetal interface) and some fetal organs had angiotensin-converting enzyme 2 (ACE2) receptors, which SARS-CoV-2 uses to enter cells (Li M, Chen l et al,). Some placentas of SARS-CoV-2 infected infants had SARS-CoV-2 in the fetal side (Faccetti et al.; Patanè et al; Schwartz, Thomas). But other studies found that human placentas lack ACE2 receptors and transmembrane protease TMPRSS2 which SARS-CoV-2 also uses to enter cells (Meštrović; Pique-Regi et al.). So, scientists found that intrauterine transmission went through the placenta, but they do not yet know the biochemical processes.
If a mother is exposed or infected while breastfeeding or pumping milk, she can probably safely continue to feed her infant, because breast milk includes antibodies to SARS-CoV-2. It is not yet known if breast milk can transmit SARS-CoV-2. She should clean her breast or pump to prevent transmitting this virus via those surfaces (Sullivan, Thompson).