Human infection with SARS-CoV-2, a novel coronavirus of probable zoonotic origin , is characterized by a spectrum of clinical conditions, ranging from mild upper airway respiratory symptoms to severe life-threatening pneumonia. In Italy, in February 2020, with the approach of the favorable season for the development of mosquitoes, the COVID outbreak has aroused concern in public opinion and health authorities for a potential transmission of the disease through mosquito bites. This concern was also justified by the detection of SARS-CoV-2 in the blood of some patients with percentages variable of positivity in human sera and/or whole blood from 8%  to 40% .
Among mosquito species, Ae. albopictus and Cx. pipiens are widespread in the country and represent efficient vectors of some of the most relevant and well-known arboviruses such as chikungunya and West Nile viruses.
In addition, the Ae. albopictus, due to its marked aggressiveness and anthropophilia, the wide diffusion at very high local densities and the trophic diurnal behavior, could represent a powerful mechanical vector of SARS-CoV-2 in an urban environment, especially in presence of a high viral circulation among the population and in absence of mosquito control activities, suspended during the lockdown. Starting from these assumptions, we tested the potential vector competence for SARS-CoV-2 of the most common and widespread mosquito species in Europe, Ae. albopictus and Cx. pipiens, using a membrane feeding system to simulate a more natural blood meal. SARS-CoV-2 genome was detected in the bodies of both species after 3 dpi, and at 7 dpi in the body of only one Ae. albopictus specimen, at a very low viral titer. The decreasing trend of the viral titer in the tested mosquitoes, at the different collection times, shows that the virus is progressively digested. A blood meal is digested by a mosquito on average after three days, but in many species this phase can last longer and exceed 5 days , like in one on the mosquitoes of our study, in whose body viral RNA was detected on the seventh day after the infectious meal. In particular, viral RNA was detected at a very low titer in the mosquito body, while it was undetectable in the wings and legs. This result show that SARS-CoV-2 did not spread to the mosquito haemocele, but remained confined to the midgut, indicating that the localization and its decrease are attributable to the digestion process and activation of the mosquito immune system.
As expected, the analysis of Ae. albopictus and Cx. pipiens larvae and adults, born from eggs deposited by potentially infected mosquitoes, demonstrated the absence of vertical transmission of the virus.
In addition to biological transmission, mechanical transmission of a pathogen is possible in nature . For this reason, we also investigated the possible mechanical transmission of SARS-CoV-2 by the Ae. albopictus, a mosquito species with a diurnal and peculiar trophic activity which, if disturbed, can make short and frequent meals on the same or different hosts. Ae. albopictus females, that had partially fed on the infectious blood and, soon afterwards, completed the meal with virus-free blood, were all positive confirming the ingestion of viral particles by the mosquitoes. On the contrary, virus-free blood was analyzed and no viral genome was detected. This suggests that mosquitoes, partially engorged with a first infectious blood, are unable to mechanically release the virus, immediately after, biting an uninfected host.