Antibody response following smallpox vaccination and revaccination

Three groups of post-vaccination sera were studied for vaccinial antibody by precipitation, haemagglutinin-inhibition, complement-fixation and neutralization tests. All sera were negative by precipitation and many by haemagglutinin-inhibition and complement-fixation tests, but most showed neutralizing activity at serum dilutions of 1/10 or higher. The differences in antibody titres between the three groups of sera were most probably related to the past history of revaccination.

This investigation was supported in part by Public Health Service Grant AI–1632–16 VR from the National Institute of Allergy and Infectious Diseases, by the World Health Organization and by the Marcus T. Reynolds III Fund.

Antibody response in non-haemorrhagic smallpox patients

Two hundred and sixteen sera from 151 patients suffering from smallpox (non-haemorrhagic) were examined for antibody by precipitation in agar gel, by haemagglutinin inhibition (HI), complement fixation (CF) and neutralization tests. Most of the patients were adults and the majority had been vaccinated earlier in life. HI and neutralizing antibodies showed rising titres from the 6th day of illness while the majority showed precipitins and CF antibodies from the 8th day. The results of the precipitation-in-agar-gel tests are in marked contrast to the findings in healthy vaccinated and revaccinated individuals, none of whose sera gave a positive result for antibody by this technique. In unvaccinated patients the antibody response was frequently delayed and the titres lower than those attained by the previously vaccinated patients. There was no exact correlation in antibody titres obtained by the four methods of measurement, HI antibody, in particular, reaching in some cases relatively high titres when other tests showed low titres. It is suggested that with the methods and materials used, a positive precipitation test in agar gel, a CF titre of 1/20 or more and an HI titre of 1/80 or higher in a single specimen of serum would be suggestive of recent smallpox infections. Such a result might be of special value in the retrospective diagnosis of missed cases and in the detection of minimal or subclinical infections.

This investigation was supported in part by Public Health Service Grant AI–1632–16 VR from the National Institute of Allergy and Infectious Diseases, by the World Health Organization and by the Marcus T. Reynolds III Fund.

Haemorrhagic smallpox

In practically all acute fulminating smallpox infections—haemorrhagic type I cases—there is severe viraemia with 104 or more infective particles of virus per ml. of blood. In most of these patients soluble antigen can be demonstrated in serum by precipitation in agar gel tests, or by the complement-fixation technique. In late haemorrhagic cases (type II) the degree of viraemia is less and soluble antigen is less often demonstrated in the blood. Five of forty type II patients recovered. The majority of the 77 patients studied were adults and bore scars of previous vaccination. Thirteen were pregnant women and 10 of these suffered from type I infections.

The antibody response in patients who survived 6 days or longer as determined by the estimation of precipitins, CF antibodies and neutralizing antibodies in serum, was considerably less than that seen in non-haemorrhagic smallpox patients.

In acute fulminating smallpox infections, the finding of virus or soluble antigen in the blood is of value in establishing the diagnosis. Soluble antigen is usually found in the blood of patients suffering from severe viraemia and with the methods used has been demonstrated only in patients who are to die of their disease. Haemorrhagic smallpox represents a generalized virus infection of unusual severity in patients who show little resistance to their infection. The cause of this unusual susceptibility is unknown but there is little evidence that specific allergy to the virus is a feature of this form of the disease.

This investigation was supported in part by Public Health Service Grant AI–1632–16 VR from the National Institute of Allergy and Infectious Diseases, by the World Health Organization and by the Marcus T. Reynolds III Fund.

Conjunctivitis and subclinical infection in smallpox

Attempts were made to demonstrate variola virus in the conjunctival exudate of 84 smallpox patients who developed conjunctivitis in the acute stage of the illness or during convalescence. Variola virus was isolated from 60 but not from the remaining 24. Of the 64 from whom virus was isolated the conjunctivitis developed from the onset up to the 15th day of illness. From conjunctivitis developing later virus was not recovered. In some patients who developed conjunctivitis early in the disease we failed to recover virus from the conjunctival exudate.

Of 55 close family contacts who stayed in hospital with smallpox patients four developed smallpox. In 21, conjunctivitis but no other illness developed. From 12 of these, variola virus was recovered from the conjunctival exudate and four of these 12, who were further investigated, showed after the appearance of conjunctivitis antibody titres similar to those seen in typical smallpox cases. From nine of the contacts who developed conjunctivitis virus was not recovered. One of these had antibody titres in serum collected before the onset of conjunctivitis which indicated recent smallpox infection. In another there was a marked antibody rise during her hospital stay although examination of conjunctival exudate on three separate occasions failed to yield variola virus. Twenty-six family contacts who developed no illness in hospital had antibody determinations made on sera collected soon after admission to hospital. In eight of these antibody titres were such as to indicate recent smallpox infection although there were no signs, in the form of scarring, or history of recent smallpox infection. These findings have been discussed in relation to the occurrence of minimal and subclinical infection in close family contacts of smallpox patients.

This investigation was supported in part by Public Health Service Grant AI–1632–16 VR from the National Institute of Allergy and Infectious Diseases, by the World Health Organization and by the Marcus T. Reynolds III Fund.