People with no symptoms transmit more than half of all cases of COVID-19, according to a model developed by researchers at the Centers for Disease Control and Prevention (JAMA Netw Open, Jan 7, 2021;4(1):e2035057). This reinforces the importance of following the CDC’s basic guidelines: wear a mask, wash your hands, and stay socially distant.

The clinical trials for the mRNA vaccines (Pfizer and Moderna) did not show whether vaccinated people are still able to spread the coronavirus (Washington Post, January 7, 2021). “Whether vaccines stop coronavirus transmission is not yet certain . . . so it is important to keep testing people.”

Data on When Infected People are Contagious
• A negative test seven days after exposure possibly rules out infection.  A study from Vermont found that one third of contacts who tested positive for COVID-19 seven days after exposure were asymptomatic and all of those who tested negative on day seven also tested negative 8-14 days later (CDC’s Morb Mortal Wkly Rep (MMWR), January 8, 2021, 70(1);12-13).
• You usually develop symptoms 4-7 days after exposure. A study of 1800 quarantined college athletes found that most had no symptoms, were infected at social gatherings, and developed positive cultures 4-7 days after exposure (MMWR Weekly, Jan 2021;70:7-11). The average infection occurred 3.8 days after exposure and had disappeared 10 days after exposure.
• In-person classes increase infection rate significantly.  Twenty-two U.S. counties that instituted remote instruction 21 days after college classes started had a 17.9 percent reduction in COVID-19 cases in the next 21 days. Counties that continued to have regular in-person classes had a 56 percent increase in COVID-19 cases in the next 21 days. (MMWR Weekly, January 8, 2021;70(1);14-19).

Progress Toward Herd Immunity
In the second week of 2021, about nine million people (2.7 percent of the U.S. population) have received at least a first dose of a vaccine, while about 22 million (about 6.6 percent of the population) have had COVID-19 (New York Times, January 11, 2021). We have a long way to go to reach herd immunity, when 75-85 percent of the population is immune by having had a vaccine or the disease.

My Recommendations
To protect yourself and to help limit spread of COVID-19, whether or not you have been able to get a vaccine:
• Wear a face mask any time you expect to be close to people who are not members of your household.
• Try to stay at least six feet away from other people.
• Wash your hands frequently or use hand sanitizers.
• Try to avoid indoor places where people congregate.
The CDC recommends getting routine testing, a 10-day isolation of persons with COVID-19, and a 14-day quarantine of people identified as close contacts of persons with confirmed COVID-19.