The Department of State’s Visa Bulletin for February 2019 notes the following concerning employment-based immigrant visa number availability on a monthly basis through May:

EMPLOYMENT-based categories (potential monthly movement)

Employment First:

WORLDWIDE (most countries): Up to two months.
China and India: Up to one month.

Employment Second:

Worldwide: Current for the foreseeable future.
China: Up to three months.
India: Up to one week

Employment Third:

Worldwide: Current
China: Up to three weeks.
India: Up to three months.
Mexico: Current
Philippines: Rapid movement to generate demand.

Employment Fourth: Current for most countries.

El Salvador, Guatemala, and Honduras: Up to one week.
Mexico: Rapid forward movement until limit is reached.

Employment Fifth: The category will remain “Current” for most countries.

China-mainland born: Up to one week.
Vietnam: Up to three weeks.

The above final action date projections indicate what is likely to happen on a monthly basis through May. The determination of the actual monthly final action dates is subject to fluctuations in applicant demand and a number of other variables. It is also important to remember that by no means has every applicant with a priority date earlier than a prevailing final action date been processed for final visa action, with applicants often processing at their own initiative and convenience.

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