Important travel and coronavirus information has just been updated by the U.S. Department of State for Travel Advisory for Mexico.

Effective January 26, 2021 all airline passengers to the United States ages two years and older must provide a negative COVID-19 viral test taken within three calendar days of travel. Alternatively, travelers to the U.S. may provide documentation from a licensed health care provider of having recovered from COVID-19 in the 90 days preceding travel.  For more information check the CDC Frequently Asked Questions website.  This requirement does not apply to travelers entering the United States by land or sea or to children under two years of age.  It applies to U.S. citizens, as well as foreign nationals, regardless of vaccination status.

Antibodies are soldiers deployed by the immune system in response to a foreign invader—in this case SARS-CoV-2. It usually takes the body two to four weeks post infection to test positive for antibodies. And it is still unclear exactly how long COVID-19 antibodies persist in the body, the estimates from the medical community vary from three to nine months.

Although rapid antibody tests are good at catching people who have high viral loads, it isn’t yet clear what the viral threshold must be to thwart contagiousness. The COVID-19 viral load seems to peak in the early phase of the infection, but viral RNA can stick around for weeks or even months, which is why you can test positive for COVID weeks after you have been exposed. With the new State Department travel guidelines this could cause a big problem.

Where antibody testing will come in handy is in estimating the spread of COVID-19 at a population level—for instance, what percentage of the population and what ethnic groups have contracted COVID-19, as well as in measuring the durability of vaccine responses.

COVID-19 Information for U.S. Citizens in Mexico | U.S. Embassy & Consulates in Mexico