In the 2019–2020 influenza season all regular-dose flu shots and all recombinant influenza vaccine in the United States are quadrivalent.In November 2019, the FDA approved Fluzone High-Dose Quadrivalent for use in the United States starting with the 2020–2021 influenza season.In February 2020, the FDA approved Fluad Quadrivalent for use in the United States. In July 2020, the FDA approved both Fluad and Fluad Quadrivalent for use in the United States for the 2020–2021 influenza season. Medical uses
The U.S Centers for Disease Control and Prevention (CDC) recommends the flu vaccine as the best way to protect people against the flu and prevent its spread. The flu vaccine can also reduce the severity of the flu if a person contracts a strain that the vaccine did not contain. It takes about two weeks following vaccination for protective antibodies to form.A 2012 meta-analysis found that flu vaccination was effective 67 percent of the time; the populations that benefited the most were HIV-positive adults aged 18 to 55 (76 percent), healthy adults aged 18 to 46 (approximately 70 percent), and healthy children aged six months to 24 months (66 percent). The influenza vaccine also appears to protect against myocardial infarction with a benefit of 15–45%. Effectiveness
A vaccine is assessed by its efficacy – the extent to which it reduces risk of disease under controlled conditions – and its effectiveness – the observed reduction in risk after the vaccine is put into use.
The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. There are two openings of the thoracic cavity, a superior thoracic aperture known as the thoracic inlet and a lower inferior thoracic aperture known as the thoracic outlet. The thoracic cavity includes the tendons as well as the cardiovascular system which could be damaged from injury to the back, spine or the neck. Structure
Structures within the thoracic cavity include:

structures of the cardiovascular system, including the heart and great vessels, which include the thoracic aorta, the pulmonary artery and all its branches, the superior and inferior vena cava, the pulmonary veins, and the azygos vein
structures of the respiratory system, including the diaphragm, trachea, bronchi and lungs
structures of the digestive system, including the esophagus,
endocrine glands, including the thymus gland,
structures of the nervous system including the paired vagus nerves, and the paired sympathetic chains,
lymphatics including the thoracic duct.It contains three potential spaces lined with mesothelium: the paired pleural cavities and the pericardial cavity. The mediastinum comprises those organs which lie in the centre of the chest between the lungs. The cavity also contains two openings one at the top, the superior thoracic aperture also called the thoracic inlet, and a lower inferior thoracic aperture which is much larger than the inlet.