Why Are There so Few COVID Reinfections in Korea?

Some 97 percent of new COVID cases in the first week of July were people who were infected for the first time, while only 2.88 percent were reinfections.

In the U.S. and U.K., reinfections account for 10 to 20 percent of infections, but here the proportion is much smaller. The BA.5 subvariant, which is the dominant strain now, is the same Omicron-induced recombinant as the BA.1 and BA.2 subvariants which led new infections early this year, so people who caught it earlier already appear to have developed some immunity.

Antibodies that were formed after infection tend to be highly effective in preventing reinfection for three or four months, so the 14 million people who caught the disease during the previous peak in March and April still appear to have some immunity left.

Cumulative coronavirus infections in Korea stand at 18.93 million or 37 percent of the country's population. That means more than six out of 10 Koreans have not yet been infected. Weekly cases have doubled from 59,000 in the last week of June to around 111,000 in the first week of July and again to 230,000 in the second week. An estimated 200,000 cases reported last were first-time infections.

Vaccine immunity is the only protection for those who have not had COVID yet, and that is weakening as uptake for second booster jabs remains low. "Most of these people got their last vaccine shot more than three months ago and preventive effects are unlikely," a health official said.

But existing vaccines are at any rate largely ineffective in preventing infection with the BA.5 subvariant. A new vaccine may be a long time coming. Moderna promised roll out before the end of August an Omicron vaccine that has 6.3 times more preventive power against the BA.5 variant.

Other major pharmaceutical companies have also begun developing vaccines that are effective against BA.5 and BA.4, but it is uncertain when they will become available.

Immunity from previous infection could also be on the wane. The proportion of reinfections has grown fourfold from 0.59 percent in early May to 2.88 percent in early July.

On Thursday morning the daily tally of new coronavirus infections stood at to 71,170, over 70,000 for the third day in a row.