Chances of contracting COVID-19 in a flight are relatively low if an airline vigilantly follows mask mandates and practices social distancing.
While traveling can still spread the virus, the risk of catching the coronavirus is relatively low during a flight as long as public health procedures such as physical distancing and masks are enforced, according to Kaiser Health News. As directed by the Center for Disease Control, airplanes are also equipped with HEPA air filters that "capture 99.97 percent of airborne particles and substantially reduce the risk of viral spread." Furthermore, air cabins frequently change the air within them (about 10-12 times per hour) which is better than the air quality of a normal building. Because of the high air exchange rate, there's lesser chances of catching COVID-19 from someone several rows away, but it is possible to get it from someone close by in a flight. If an airline isn’t keeping middle seats open or enforcing mask use, flying can actually be rather dangerous.
A CNN article recounts an incident where 328 passengers had to get tested for coronavirus as their flight from the US to Taiwan had been carrying 12 passengers who were already infected at the time. Fortunately, all other passengers and crew members tested negative. The article also refers to a study by Massachusetts Institute of Technology's professor Arnold Barnett which claims that based on short flights in the US where the aircraft has three seats on either side of the aisle -- assuming everyone is wearing a mask -- the risk of catching the virus on a full flight is just 1 in 4,300.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.