Peer Reviewed


COVID-19 Roundup: Antibody Levels and Prior Infection, Monoclonal Antibodies, and Ivermectin

Antibody Levels, Vaccination, and Prior Infection1

Fully vaccinated individuals who have been previously infected with COVID-19 may have more durable antibodies than those who were vaccinated but were not previously infected, according to the results of a recent longitudinal study.

A total of 1960 health care workers from the Johns Hopkins Health System provided serum samples at least 14 days after receiving their second dose of an mRNA vaccine from June 2020 to September 2021. Prior infection was determined through a SARS-CoV-2 polymerase chain reaction (PCR) test before their first vaccine dose.

The results indicated that 3.7% (n = 73) of vaccinated health care workers had evidence of a previous COVID-19 infection. Of the previously infected participants, 41 had a positive PCR test result 90 or more days before vaccination while 32 individuals had a positive PCR test result less than 90 days before vaccination.

“Health care workers with prior SARS-CoV-2 infection followed by 2 doses of mRNA vaccine (3 independent exposures to spike antigen) developed higher spike antibody measurements than individuals with vaccination alone,” the researchers concluded. “Consistent with work comparing extended vaccine dosing intervals, the study showed that a longer interval between infection and first dose may enhance the antibody response.”

REGEN-COV and COVID-19 Outcomes2

REGEN-COV, a combination of casirivimab and imdevimab monoclonal antibodies, may reduce the risk of hospitalization and death in individuals with COVID-19, according to the preliminary results of an adaptive trial.

A total of 2519 patients with at least 1 risk factor for severe disease were included in phase 3 of this trial. The participants were randomly assigned to receive 2400 mg of REGEN-COV, 1200 mg of REGEN-COV, or placebo in a single dose.

Of those in the 2400-mg group, hospitalization or death related to COVID-19 occurred in 18 individuals, compared with 62 of the 1341 patients in the placebo group. These adverse outcomes occurred in 7 of 736 patients who received 1200 mg of REGEN-COV, compared with 24 of 748 patients who received placebo.

In individuals who received REGEN-COV, the median time to resolution of symptoms was 4 days shorter than those who received placebo. Further, in both groups who received REGEN-COV the viral load was reduced faster than placebo.

Toxic Effects of Ivermectin3

The use of ivermectin to prevent and treat COVID-19 infection may cause toxic effects such as confusion, seizures, and hypotension, according to the results of several recent case studies.

A spike in calls throughout 2020 and 2021 was reported by the Oregon Poison Center. Of the reported doses in August 2021, 17 of the total 21 doses were from veterinary products and were not approved for human use.

Hospitalization was required in 6 of these individuals. Symptoms of those who were hospitalized included gastrointestinal distress, ataxia and weakness, hypotension, and seizures. Individuals who were not hospitalized reported gastrointestinal distress, dizziness, confusion, vision symptoms, and rash.


—Leigh Precopio



  1. Zhong D, Xiao S, Debes AK, et al. Durability of antibody levels after vaccination with mRNA SARS-CoV-2 vaccine in individuals with or without prior infection. JAMA. Published online November 1, 2021.
  2. Weinreich DM, Sivapalasingam S, Norton T, et al; the Trial Investigators. REGEN-COV antibody combination and outcomes in outpatients with COVID-19. N Engl J Med. Published online September 29, 2021.
  3. Temple C, Hoang R, Hendrickson RG. Toxic effects from ivermectin use associated with prevention and treatment of COVID-19. N Engl J Med. Published online October 20, 2021.