SARS-CoV-2 Infection in Vaccinated Health Care Workers

To the Editor

Keehner et al.¹ (Sept. 30 issue) report a resurgence of SARS-CoV-2 infections, mostly with the delta variant, in vaccinated health care workers in San Diego in July. The reduction in vaccine protection was allegedly due to emergence of the delta variant, waning immunity, and the end of masking requirements in California.

Table 1. Table 1. SARS-CoV-2 Infection in Vaccinated HCWs at Fondazione IRCCS Policlinico San Matteo, Pavia, Italy (February–August, 2021).

In May 2021, vaccine protection against both symptomatic and asymptomatic infection was 83% among 3720 vaccinated health care workers at Fondazione IRCCS Policlinico San Matteo, Pavia, Italy.² We did not observe a subsequent increase, but rather a decrease, in the frequency of breakthrough infections (Table 1). The delta variant appeared in Italy³ in April and subsequently became predominant. In line with data from England,⁴ our findings show a persistent vaccine effectiveness against the delta variant. Although the levels of vaccine-elicited neutralizing antibodies were reduced against the delta variant, the effect of these antibodies was not abolished.⁵ The discrepancy between our data and the data from California may reside in the persistence of an indoor masking requirement, broad population testing, and high vaccination coverage in Italy by August 31 (71% coverage with one dose and 61% coverage with two doses), all of which reduce the circulation of the virus.

Daniele Lilleri, M.D.
Carlo Marena, M.D.
Fondazione IRCCS Policlinico San Matteo, Pavia, Italy
[email protected]

Fausto Baldanti, M.D.
Università degi Studi di Pavia, Pavia, Italy

This letter was published on December 15, 2021, at NEJM.org.