World Health Organization
As of 2016, the World Health Organization (WHO) recommends seasonal influenza vaccination for:First priority:

Pregnant womenSecond priority (in no particular order):

Children aged 6–59 months
Elderly
Individuals with specific chronic medical conditions
Health-care workers

Canada
The National Advisory Committee on Immunization (NACI), the group that advises the Public Health Agency of Canada, recommends that everyone over six months of age be encouraged to receive annual influenza vaccination, and that children between the age of six months and 24 months, and their household contacts, should be considered a high priority for the flu vaccine. Particularly:

People at high risk of influenza-related complications or hospitalization, including people who are morbidly obese, healthy pregnant women, children aged 6–59 months, the elderly, aboriginals, and people with one of an itemized list of chronic health conditions
People capable of transmitting influenza to those at high risk, including household contacts and health care workers
People who provide essential community services
Certain poultry workersLive attenuated influenza vaccine (LAIV) was not available in Canada for the 2019–2020 season. European Union
The European Centre for Disease Prevention and Control (ECDC) recommends vaccinating the elderly as a priority, with a secondary priority people with chronic medical conditions and health care workers.The influenza vaccination strategy is generally that of protecting vulnerable people, rather than limiting influenza circulation or eliminating human influenza sickness. This is in contrast with the high herd immunity strategies for other infectious diseases such as polio and measles. This is also due in part to the financial and logistics burden associated with the need of an annual injection.
After removal, the fluid may then be evaluated for:

Chemical composition including protein, lactate dehydrogenase (LDH), albumin, amylase, pH, and glucose
Gram stain and culture to identify possible bacterial infections
White and red blood cell counts and differential white blood cell counts
Cytopathology to identify cancer cells, but may also identify some infective organisms
Other tests as suggested by the clinical situation – lipids, fungal culture, viral culture, tuberculosis cultures, lupus cell prep, specific immunoglobulins

Lights criteria
Definitions of the terms "transudate" and "exudate" are the source of much confusion. Briefly, transudate is produced through pressure filtration without capillary injury while exudate is "inflammatory fluid" leaking between cells.Transudative pleural effusions are defined as effusions that are caused by systemic factors that alter the pleural equilibrium, or Starling forces. The components of the Starling forces – hydrostatic pressure, permeability, and oncotic pressure (effective pressure due to the composition of the pleural fluid and blood) – are altered in many diseases, e.g., left ventricular failure, kidney failure, liver failure, and cirrhosis. Exudative pleural effusions, by contrast, are caused by alterations in local factors that influence the formation and absorption of pleural fluid (e.g., bacterial pneumonia, cancer, pulmonary embolism, and viral infection).An accurate diagnosis of the cause of the effusion, transudate versus exudate, relies on a comparison of the chemistries in the pleural fluid to those in the blood, using Lights criteria. According to Lights criteria (Light, et al.